Has your dog ever turned on you when they are acting threatening to another person or animal, this is redirected aggression? This type of aggression usually occurs when a dog is trying to get to something and cannot, they redirect the aggression from the initial person, animal or object and attack for the closer person, animal or object.
Dominant dogs often redirect onto subordinates, whether animal or human, so if your dog redirects hostility, he may be showing signs of a deeper-seated dominance-aggression problem. Contact a behavioral trainer to assist in diagnosing this form of aggression.