There are at least 2 schools of thought on this, one is that it is a form of 'victory' dance, and two, in the wild, when dogs eat they get blood on their snout (they roll around in their kill too), having blood on the snout can be a problem, for one, it masks future kills by its scent, and it also warns prey as they an smell the blood (some say it's the opposite, and that they roll around in the kill and WANT to keep the blood on them, to cloak their own smell from the next kill), the instinct is to 'wash' it off, sometimes habits like this can show up in domesticated dogs.