He thinks he's playing a game with you. If I have a leash biter, I drop the leash immediately (step on the end if I am out somewhere) and let him get bored. He learns if he bites the leash, all the fun dies. When he calms down, continue with the walk. As you are walking him, you can also anticipate his biting at the leash. If I have taught a leave it, I can look for the energy ramping up, snapping with the mouth, focus on the leash. I can deliver a sharp leave it and reward if he chose to refocus on me instead of grabbing the leash.