I've never been owned by a cat who had this problem, but I know a couple people who have. Sometimes it's a behavioral thing (be thankful they're not pissing on stuff instead is what I say about that) and sometimes it's medical. A friend of mine has a cat who is 16 years old and suffers from kidney disease. She doesn't like her medication and tends to tear the hair out of her hindquarters and the base of her tail, perhaps because she's uncomfortable and trying to make it feel better?

But skin allergies would also do it.