You start to loose traction going straight up the hill, so you step on the gas. Your tires are both spinning, but you don’t have equal traction on both sides (partly because of the differential in the transmission, partly just because the traction isn’t equal for whatever reason.) Unequal traction means one side of the car digs in, and the car starts rotating - then you’re accelerating, while pointed to one side - and you end up off in the woods.