I have a betta who had this problem as well. His seemed to be stress related. I put his tank somewhere quiet, densely planted it, added a cave, and kept it covered if I was making any dramatic changes. I also made sure the environment around his tank was extremely stable; I never left items near the tank that weren't usually there, made sure cats/dogs/other fish were out of sight, and made sure the location was low traffic and low noise. He's doing spectacularly now, but sometimes a stressful situation - including poor water, irregular feeding, etc. - will make him start up again.
Unfortunately it's chronic with some bettas. I have one that will go for weeks and be fine and then suddenly he whips around and bites off a mouthful of tail. 
I know he gets plenty to eat. He's a little pig and eats everything that comes into his tank. I have heard it comes from boredom, but I also tried changing things around in his tank (that keeps him busy for an hour or so), putting ghost shrimp in with him (he ate them), putting ADF's in with him (he tried to eat them, too), and finally a snail that is as big as he is. I also spend time interacting with him every morning and evening. He's better entertained than I am! I've also heard that it could be water quality, I have two other bettas in the same water and they don't tail bite. 
Yeah, I tried all that with mine, too. Kept him in the bedroom away from cats, kids, and company, where it was quiet and darker than the rest of the house. It made no difference. He still does it every two or three weeks. 
