My parents' Great Dane has had this problem off and on for about 6 years! From what they've been told, it seems most vets are stumped as to why it starts, why it stops, and what exactly the dog gets out of the behavior. There's lots of theories and speculations, but that's it. I'm just glad I've never had a dog that does it! (Kobie and Eli love to snack on "kitty crunchies" but that doesn't count, right? )