Dogs have many weird behaviours .. I knew a dog once that loved to chew on rocks .. He did it so much that he actually wore down his teeth.
I think you might get the best information from your vet. He may need a 'check up' to see if he may be lacking a mineral of some sort that he is not getting from his food source.
or .. it may be more of a stress symptom. Is he getting enough exercise and attention? Is he locked up too long? There could be many reasons or even combinations of reasons. You are going to have to consider all factors involved in his care.