My 4 year old does this, and its causing problems, because while he's done it since he was a yearling, he is not doing in undersaddle as well. Teeth have all been done ect. He's even been scoped for ulcers. Nothing. 2 vets actually think its behavioral; kind of like how some people constantly tap things, he constantly opens his mouth.
I'm actually taking him in tomorrow to have his head and jaw xrayed to see if there something that we can pick up.