And you're right, since they probably just assumed that their buddy had slipped on loose rocks, they never would've thought to look for anything biting through the rope and/or pushing them down the mountain. And then you're right, the goat did take advantage of the last one's feeling of comfort and victory and everything when he pushed the big ol' boulder into him.
Maaaan, I think that evil goat's smarter than we give him credit for!