That's an excellent question, and I hope some materials engineer can give us a good answer.
They did indeed disintegrate, I've seen it with my own eyes many times.
It wasn't because they were nitrocellulose-based, as Fender didn't use nitrocellulose for pickguards after October of 1964, or so FMIC has told me.
In a related question, I should like to know why Fender pickguards
still shrink so fatally after all these years.
