The sheep behaviour is scripted, but not the way you think. It will always try to increase the distance between it and the player, and if two stars are at the same distance away from the player, then it will prioritize a star according to a list that I arbitrary set (especially not to get too easy levels, it will just prevent the player to find too easy solutions). So the main mechanic is to use that distance to force the sheep going where you, as a player, want.