Oh, any number of things. I would say most members here have a biter do it for different reasons! My first VT bit bc of a bubbler agitating the water, then he bit for habit. My current VT bit for who knows what reason but stopped when I got a larger tank and divided it with Bowser.
Some bite bc the water is too strong in current (from a filter), then others bite because there are not enough plants, not getting fed on time, water is too cold, tank doesn't get enough light.. Anything you can think of. Sometimes it's hard to find out why they bite, sometimes its as easy as turning on the light and feeding them right away.