[JRTJH]

While not the "only reason" one of the contributing factors is distortion caused by heating/cooling. Think of a rubber balloon. Blow it up, let out the air and compare the surface to a balloon that has not been inflated. Significant difference in surface distortion caused by the stretching. There is some of that same distortion, some "oil canning" some moisture expansion, some base support separation and some "bad luck".

I don't think there's any one reason, but a combination of several factors that, when they all occur, causes the result. And yes, that's one of the reasons the "more expensive models" have moved away from FILON front and rear caps. They are on both ends of "entry level", fiberglass cap on the front and FILON on the rear of "premium level" and are replaced by fiberglass caps on front and rear on the "luxury level" trailers.