Connie, I have seen a similar case on the dog whisperer of resource guarding, and my first dog used to behave the same way so the kind of guarding you mentioned could be the explanation indeed. Luckily the GSD that I have now does not behave that way.

Trevor, you may be right about the tension that my dog senses on the leash which increases his need to protect himself, but once he escaped from the yard when he was unleashed and attacked a pitbull from the front, completely unexpected, so he does this on and off leash. My tension may be wrong, but you see, there is a good reason for this, because although I have an insurance, there are some people here (like everywhere) who like to get even in less civilized way after dogfights. For example: today a boy walked with his pitbull unleashed on my block in front of a house with an open yard with a small dog. From what I heard the pitbull atacked the small dog in the yard. The owner of the small dog followed the pitbull after the fight and stabbed the pitbull multiple times until the dog died. I am not a fan of pitbulls but I find this very sad because this could have been prevented if the pitbull was on a leash and the yard of the other owner was closed. Another problem here is that a lot of people here let their dogs run free, unleashed, so sometimes they try to atack my dog in a pack when I am jogging with my dog. However after the incident in my neighbourhood I will think twice before I take my dog with me when I go jogging, even if he stays behind whimpering and yelping.