I think it's one of a Corgis more charming traits. I had one Corgi that was a back sleeper and another that wasn't. Gromit is a back sleep and it makes it just a bit more fun to own him.
We always say that it looks like someone gassed the house because the corgis and our two cats all seem to sleep in these sprawled out positions. Yet another reason to be a corgi person!
Definitely a corgi trait...don't mine the mess in the background--we are redoing the house...
but Seanna sleeps like this all the time, and sleeps so hard that you can't move her. I have to pick her up like a baby and move her over...