Wait..... isn't that cribbing? (I'm probably wrong)
To me it is more likely that it is due to stress or boredom.
Bryce is right, there are many reasons. Your horse can have a discomfort in the mouth and he must chew because it relieve his pain or to try to eliminate it. It can be due to anxiety. When does he do this? When you brush him? When you bring him to another place? It can also be because he's bored or just to play. Say "No!!!" every time he takes the tether in his mouth and try to find him toys :)
So many of the young horses I ride do this. It can be due to any number of reasons. Some horses are just more "mouthy" and enjoy having something to chew on. Sometimes it is because they are bored or anxious.