Tom -- That's the only thing (unintentional pulling) that makes much sense to me. I mean that rubber doesn't know where it is, so why should it decided to so noticeably shrink only after it's actually installed on the car?
Other possibilities might include exposure to direct sunlight (of which my car gets little), significant temperature swings, etc. However, environmental exposure seems a bit doubtful in terms of being the cause of the significant shrinkage I've noticed.
Oh well, we'll just try it again and see what happens.
Tom in Boise