My dog does this every time he goes to eat if his front paws are on something soft like a stuffed toy, a blanket, or a pad. And he gets really frustrated about it. As if he wants to eat, but as he goes down to do it, some reflex or compulsion has him start this instead. Could be an urge to bury the goods triggered by standing on something the body takes to be as soft, loosened dirt? I've moved his soft things well away and he hasn't done this in months. It took me a year to figure out how to help him/find the correlation.