I have seen them do this also.I think they dig for insulating around there bodies and maybe to help hide the contrast of there dark coat on the white snow.I usually see beds like this inwoods that are very easy to see through when there is snow on the ground.So maybe its a camo thing.If the snow is deep I usually dont see dug outbeds.If the snow is only a couple of inchs I think they try to build it up around them.I dont know for sure good question.