A quarter of a century in the freezer is pushing your luck I would suppose. I wouldn't want to be in there for that long :]
Could be any number of things that caused it like cosmic rays (always want to go read a Philip Dick story whenever that term comes up), moisture had seeped in through a teeny space, etc. You will never, ever know what went wrong. Just don't do it again is the best practice.
Myself, I dislike putting anything in the freezer besides food, and even that I try not to do. It's bound to cause cellular damage. While it's true that some films are unique, and when they're gone they're gone, that's my policy. They're gone. Lots of good stuff out there to still shoot.