PLEASE USE YOUR FULL NAME OR THE POST WILL BE DELETED. INAPPROPRIATE LANGUAGE WILL GET YOU BANNED. PLEASE BE CONSCIOUS THAT THESE ARE PUBLIC POSTS. PLEASE BE CONSIDERATE OF CHILDREN THAT MAY BE SIGNED ON.
I've seen dogs do that to keep as much weight off their shoulders as possible because they hurt. Might not be the problem wit your dog but I would guess that is what it is.
It's referred to as being rabbit ended. It's like the dog is running like a rabbit with the hind end tucking under when it moves. There is nothing you can do for it. Simply a deformity.