Why did the brakes go like this?

[blown motor]

This is on a Mazda 6, not mine. All four brakes were replaced by a Mazda dealer 1 1/2 -2 years ago. The front pads have a lot of life left and the rotors are smooth and shiny on both sides. Outer rear pad is getting thin and inner pad isn't too bad. Both pads are ribbed. Rotors look like crap! What causes them to go like this?

[JH27N0B]

I've had issues with premature brake life in several of my cars with disc brakes. My theory is rusting of the rotors.
I live in the rust belt and I've only had issues with my cars that aren't garaged.
Last year the rear brakes in my '14 Dart were completely shot with under 50,000 miles on them. One pad was down to the metal backing.  I've gotten 100,000 miles out of a set of brakes before, so I know it isn't my driving style.
Was your car parked outside?  Do you have mag type wheels where you can see part of the rotor without removing the wheel? When you first start out in the morning when you put in gear, do you feel a bit of hesitation before the car "breaks loose"? 
If you can see the rotor, look at it sometimes when the car is parked, after you do the brake job. Can you see a faint outline of a brake pad? If you feel it "break loose" when first moving after the car sat, and can see faint pad outlines in the rotors, your pads are lightly rusting to the rotor. Some metal from your rotor is breaking off every time you feel it "breaking free" on starting off!
I used to have a '95 Ram I used as a second car which I parked outside.  Its front brakes would usually be shot with not much over 10,000 miles on them and the rotors would look like the surface of the moon!
I worked at a brake manufacturing company at the time and got brake parts with my employee discount, but it would still cost me about $90 for loaded calipers and new rotors.
I griped to my engineering and product manager colleagues but best answer I would get is you need to drive your truck more. I'd on average drive it once a week.
And yes, you could see pad shaped markings on the rotor surface all the time.
I finally got the idea to try ceramic pads and that helped some, maybe 15,000 miles instead of 10,000 miles on a set of front brakes. I got over 75,000 miles out of the rear drum brakes on that truck.
Use coated rotors, and ceramic pads is about all you can do. Or move to a dry climate and park in a garage! 

[blown motor]

It's not my car so I don't know about the "break loose" in the morning. I think it's often parked outside in the summer and inside in the winter. Yes it is an open spoke type rim where you can see the rotors.
Perhaps some of your theories are correct but then why are the rears crap and the fronts so good still?

[JH27N0B]

One possible factor is the parking brake. I would set the parking brake on my Dart every night, but have stopped doing that.  That's not necessarily a help though. Many rear caliper designs incorporate an internal brake shoe design for the parking brake instead of using the caliper pistons to push the pads into the rotors to keep the car from rolling away. I don't know if my Dart uses internal shoes or not, but my driveway is flat so I figured it can't hurt, and no risk of my car rolling downhill and into another car or out into the street. 
For some reason in '94-'97 Rams, Dodge used a crappy off the shelf Delco brake system that already had a bad reputation on Chevy and GMC trucks it was already used on.  The pads were apparently lightly touching the rotors when parked which made the pads susceptible to rusting to the rotors. That contributed to why I had problems with its front brakes.  But otherwise it's a problem that I've seen mostly in rear disc brakes.
I don't claim to 100% have the answer, but it's a frustrating problem I've dealt with starting in the 90s with my Ram.  Cars I've kept garaged and not driven when the streets are salty I've never had a problem at all. Including daily drivers.
So my theory of corrosion is well grounded, but not necessarily proven.
If the choice was still available, I'd opt for rear drums on cars and trucks exposed to harsher climates. But rear disc is pretty much universal now.
I recommended coated rotors, but I have to admit the rear brakes on my Dart that were shot with less than 50K miles, around 4 years, I'd put coated rotors on it.  They looked brand new, for about 2 years, then they didn't!
Maybe some brands are better than others. I used Centric.
When I worked for the company that made Raybestos and NAPA United brakes, until 2005, they didn't have coated brake rotors.  Now everyone sells them, it seems apparent rotor corrosion is a problem that the brake manufacturers came up with this coated rotor product to try to address.

[dodj]

Pad thin on one side and thick(er) on the other is a caliper that cannot move on the pins. The caliper itself is functioning, but the pins are seized.

No idea on the grooving unless maybe they are cross drilled rotors...and that still doesn't really explain it....

[MoparLeo]

I believe that the rotors were scored before the pad replacement. Most dealer do not include rotor resurfacing in a basic brake pad replacement.
More wear on one side pad (usually the inside, piston side) is normal with a caliper that only has piston(s) on one side of the rotor. That is why high performance disc brake setups have pistons on both sides of the rotor and the caliper is fixed instead of floating.
The pictures showing the rotors looks like the car was parked for a long period of time and the rotors were wet when parked.
I have seen this exact condition many, many times over the years on customer cars. Nothing unique.

[blown motor]

The pictures showing the rotors looks like the car was parked for a long period of time and the rotors were wet when parked.
I have seen this exact condition many, many times over the years on customer cars. Nothing unique.

But then why aren't the fronts like that also? The front rotors are perfectly smooth and shiny.

[MoparLeo]

The other rotors were not damaged before. The grooved rotors show heavy rust pitting. Where did that come from ?? You asked the question based on it was someone elses car. You may not know all the history.
Based on what you are saying about the other rotors being shiny and smooth, have you ever worn a set of brakes down to the metal ?The rotor will looks exactly like the damaged one. Obvious the pad will be softer than the rotor so the only way the rotor can be worn ;like that is " metal to metal" damage.
It is impossible to diagnose a brake issue based on a picture of only 1/2 of the brake system and no previous knowledge of the brakes when originally serviced.
You say all 4 wheels had the brake pads replaced 1 1/2 years ago. What is the year of the car and the mileage ?
Why were all 4 wheels serviced ? Front and rear brakes seldom wear out at the same time. Normal brake wear is 1 1/2 to 3 times faster wear on the front brakes compared to the year when equipped with 4 wheel disc. On drum rear and disc front I have seen the fronts need replacing up to to 4 times before the rear neede service.
I suspect that the front rotors have been replaced. No pictures so hard to tell.
Not enough info given to make a qualified answer.
If the owner has the dealer paperwork from the service it should have the old brake measurements before the service or even the owner original reason for coming ion for the service. Very few people have brakes done before they have an issue with them like vibrations, noises etc...

[Dmod1974]

Sticking rear calipers.....  They're grooved and rusted because the caliper wasn't able to float and apply/release like it should.  The uneven pad wear confirms that.  Being in the rust belt, virtually every rear brake job I've done on a 4 wheel disc vehicle had sticking/seized slider pins causing the pads to wear unevenly to the point of requiring replacement even though 2 or 3 of the pads were still fine.  IMO, rear drums were more reliable in this climate at least.  Other than the occasional leaky wheel cylinder or seized self adjuster, they tended to wear a lot more evenly and have less issues with corrosion.

[blown motor]

Sticking rear calipers.....  They're grooved and rusted because the caliper wasn't able to float and apply/release like it should.  The uneven pad wear confirms that.  Being in the rust belt, virtually every rear brake job I've done on a 4 wheel disc vehicle had sticking/seized slider pins causing the pads to wear unevenly to the point of requiring replacement even though 2 or 3 of the pads were still fine.

I get it that uneven pad wear indicates that both pads aren't being applied evenly. In this case the outer pad was the thin one which means it's being applied more. In my scientificly weak mind that says the calliper is sliding, otherwise it wouldn't be moving inward and applying the outer pad and the inner pad would be taking the wear. Sorry guys, but my mind tries to logically rationalize what is happening.
The pins were well lubricated and sliding freely when disassembled. Maybe the calliper was getting hung up somehow, I don't know. Anyways it's got new pads and rotors on it now.

[dodj]

Then the pad itself was not free to move in the caliper. When the brakes were applied, the caliper squeezed the pads against the rotor, but when released, the pad could not relax and stayed "sort of" tight to the rotor, wearing it down.

[moparroy]

I have found there is a range of quality in brake parts - dealer should use OE or eq parts but maybe they could not get them that day and bought some cheap stuff from a supplier. I prefer Raybestos E3 rotors now - their corrosion protection is good and the pads are good too.
I have had a set  of pads wear out in as little as 20kkm if they are cheap pad material - and they were cheap.
True the pins may well have been sticky - but only if the dealer did not service them properly - should not seize in as little as 18 months if they are properly serviced in the brake job (unless your buddy refused to replace them?). I lube mine every seasonal tire change now but before that I would notice that 12 months was enough to go from iffy to stiff.
I find the rear rotors more susceptible to rust - I have been told its cause they get more salt spray on them. I believe its also because they don't get cleaned as well as the front as the rears are under lower pressure. Lots of sitting and short trips makes this worse.
Parking brake on disk? is it working properly? driven on at all?

[Dmod1974]

Then the pad itself was not free to move in the caliper. When the brakes were applied, the caliper squeezed the pads against the rotor, but when released, the pad could not relax and stayed "sort of" tight to the rotor, wearing it down.

This.  Rear brakes do so little of the work that the pins tend to seize and the pad abutment clips tend to get rust jacking under them which keeps the pads from moving freely.  When they drag, the rotors get hot and grooved.  When they stick, rust forms on the rotor surface.  Both lead to the pads and rotors wearing out prematurely.  If I don't clean and lube my rear caliper sliders yearly, no matter what car, they will be sticking or seized in a year or two.

Most rear calipers nowawdays use a drum in rotor hat parking brake, so there is no problematic self adjuster ratchet mechanism tied to the actual disc brakes anymore.

[JH27N0B]

I do my own brake jobs and my rear pads showed uneven wear.  I have a tube of silicon grease I use to lube the pins and slides. When I disassemble my brakes the pins are clean and lots of fresh grease still present.  I'm skeptical that at least in my case the calipers are hanging up.
I'd be interested in knowing how often people from dry climates see this issue.  I mostly blame the brake problems I have had on all the salt they dump on the roads here every winter. Also, with my open style wheels, after the car sits out in the rain, the next day I'll see some rust on the exposed parts of my rotors and hear a little brake noise for the first couple stops when I start my drive, until that surface rust gets wiped off.
On my Dart, which I bought new and just hit 115,000 miles, the rear brakes were trash by 60K miles and then next at around 105K miles last May.
I did the first front brake job at around 85K, and the rotors didn't look too bad and the pads still seemed to have some life left in them. The front brakes on the Dart are noticeably bigger then the rear though I don't know how that factors into the life expectancy.  When my delivery came from Rock Auto a few years ago with my new front brakes, I thought they'd sent me the wrong rotors since they were about twice the size of the rears I had previously changed.  Lo and behold, they were correct!