There are different theories, one is stress, one is boredom, another is that their tail is too heavy for them.
All I know is that you can do everything right, give them the best care, and they still might tail bite. My boy has chewed up his tail twice. First time was stress, no idea what triggered him the second time.
Alter his decor, add live plants, an air stone was suggested so he would have something else to keep his attention. Keep the inside and outside of his tank "updated". Different background, etc. Other than that you can treat to help prevent fin rot.
As a side note, this is not just something that happens to fish, this sort of.. mutilation is done by humans to themselves. I naturally assume this isn't related to why bettas tail bite, for humans it's a psychological condition. Sorry if I'm off on a tangent, when I saw it in my fish, I couldn't help but think of this.