I've never seen a dog do this, is it a breed thing? Who knows, all I can say is Casey does the craziest things with her back legs. She loves to curl her back "toes" up like a baby and she always has her legs spread out like a frog about to leap.
When she was younger, she would naw on her back leg, like a pacifier or something. She never broke the skin, but man she would chew and chew it. I'm glad she doesn't do this much anymore, it made me laugh, but it also freaked me out a little.
This pose however, can last, I just laugh like crazy when I see her.....you have to admit, it is cute!