Something had to have gotten on it to soften the rubber, could be as simple as a rag soaked with laquer thinner touched it and caused it to break down. Who knows, but sorry for the loss.
First one I have seen with this kind of wear and tear. I doubt you will have much luck with GM, so I would probably just buy a replacement remote on ebay for about 40-50 bucks. Swap the electronics out so you won't have to bother paying the dealer to program it to the truck for you.