They wan't to trample the snow or grass down for bedding. In deep snow they sometimes dig out a spot to get bellow wind (or thats what we think they are doing). Here is another interesting fact that you may not know. Sled dogs do not stop to pee or poop. They do it while running or trotting. One of the older dogs my mother had adopted was going blind in her left eye and was alowed in her house due to the dogs age. The dog would spend the day walking clockwise around her kitchen island pooping and peeing whenever it needed too. It was messy but the dog was at the end of it's life so my mom just dealt with it.