My mother's cat does this too, and we've never found a solution. The vet thinks it's allergies, as others have mentioned. I know this doesn't help at all, but you're certainly not alone! Mom's kitty has done it for years and she's all right, just with spindly, bald legs sometimes. She's also a very anxious and ornery cat, so maybe that has something to do with it?

In any case, good luck with your kitty!