Elastomeric Bumper Question

[72and86]

Ive removed my rear Elastomeric bumper for restoration. Old paint was removed, it was repaired of any imperfections and primed with a body-repair grade primer by my friend. I let it sit in my garage during the winter. I doubt the temperature hit below 30F in the garage. (Connecticut winters are cold but not as bad as the other New England states.)

When I pulled it out to finish the work on it I noticed many hair-line cracks on it. The bumper was never outside during the winter, hit or fell over. It was virtually untouched all winter and now these cracks are there. What would cause this? My friend and I are baffled. He used the same primer he used on other modern flexible bumpers.

[anlauto]

Was there a lot of repair work done to it ? The one crack has material that looks pretty "thick" 

[JS29]

Some will say it happened because the bumper was off the car, allowed to flex more than normal. I don't know what primer was used, I have had good success with PPG's K36 primer with the flexible catalyst (hardener) made for flexible parts.   

[72and86]

Was there a lot of repair work done to it ? The one crack has material that looks pretty "thick" 

Not really. Just some small nicks and imperfections.