[bewitched]

Some dogs have such high food aggression/protectiveness that anything they see out of the corner of their eye will set them off: a piece of paper rustling, wind blowing, etc.. I think it's more like the "what the hell is that?!" aspect that I would liken to a cat chasing its tail...

I do have to say that on further viewing, it kind of looks like maybe they are manipulating his leg with a string or something which puts me on the fence: he's not hurting himself, he appears well taken care of, it seems to be all in good fun (not mention that it is pretty damn funny), he got a yummy bone out of it (which in itself would probably make it worth it for most dogs) and is it really any worse than something like giving a cat catnip and laughing when they go nuts...but still.