It's nothing more than bad manners and wasn't corrected a long time ago when it should of been. Drafts are no more mouthy than any other horse. I think the real problem is because of their size, the handlers tend to back off and give them space instead of making the horse back off, not crowd and run over the person. I would also suspect he is hand fed treats. If you feed him carrots or cookies put it in a feed pan and never let him frisk you for treats.