There are so many factors in tail biting.
I have had a betta who is now rehomed to a good friend in exchange for her rescue betta... His name was Crayola, and his tail grew very very long. It became double his body length, so he trims it back. It's to avoid being weighed down...
Another reason, can be boredom.
Or, they are not used to having long fins (my Shiloh has this, after being without fins since he got in a scat with the pet store cichlids/another betta),
or they are just disliking their tail because it's following them
there's so many reasons it is hard to narrow it down.
what pataflafla said is a good idea. More plants, or "toys" such as a bottle cap lid, or ping pong ball... clean and warm water, good foods (pellet, freeze dried as treats, frozen, live...) can help.
but there are always some that are just tail biters, as some people on here will probably tell you
sometimes you cannot stop it... but just keep it from being infected.