I had a shorthair, Toot, who overgroomed. At some points he would even pull the hair out of his front legs! But he was a nervous cat, partly because my boyfriend abused him as a kitten - it had been going on several weeks before I knew about it. He groomed so much, in fact, that a couple of times he got a tapeworm, but never once had a flea! He would apparently swallow them the minute they jumped on him! I could tell when something was stressing him out because the fur on his front legs would disappear. Oftentimes it is due to something that can be alleviated, but some cats are just prone to it.