Well, if he's actually carrying something and struggling to get it up that hill, it could look like he's walking on his hind legs.
Theoretically, the item he is carrying would cause him to try to stand up to keep it from dragging in the snow.
If you can only grip something with your mouth, and it's rather long like a deer carcass, you are going to have to get on your hind legs as best you can if you want to keep it from dragging in the snow.
I think the bear is sort of herky-jerky carrying some sort of carcass up the hill with his mouth. He is trying to keep it from dragging by holding it up high with his mouth and walking on his hind legs. He keeps having to reset himself and pick the the carcass up high again because it's not working out well. He can't walk too well this way and the carcass keeps ending up between his legs and in the way.
He is probably startled by the car and just trying to get out of there with his road kill as quick as he can.
Either that, or it's a clumsy fool in a cheap suit.