I've watched them in the act of creating new tracks (and causing a bit of erosion damage) on Silbury. It's a good place to observe what their motivations are as there are few variations in the land surface so what they do can't be attributed to the terrain. It seems to be a mixture of initial randomness, short cutting and follow-my-leader behaviour rather like.....umm sheep. And I guess humans create tracks in much the same way except maybe we prefer to go up slopes more diagonally, being more wimpish.