Could just be a habit or lots of energy :-) Why does it bother you? Some people "pace" when thinking, maybe you bounce your leg. Think of some way to make it more conscious if it bothers you and you want to have more control of it, or some way to put it to "good" use? Is it only one, particular leg? See if you can figure out/write down what you're thinking or feeling when you recognize it's bouncing. That's the only way you can know if it's because of anxiety?