Worms can also cause that behavior. Might want to have it checked or just keep an eye on the little guys poo.
I have a year old weimaraner that does then when he gets really excited (like when we just come home from work). He is turning his butt towards us so we can give him a scratch but he wants to see us at the same time and he ends up walking about in a you shape. Does it seem like maybe your dog is excited? I quick trip to the vet can determine if there is anything neurologic going on or any type of health issue that could be causing it.
Has the dog always done this, or is it a new thing? My lab is 9 and still does this, I think it is just his playful demeanor.