It's really very simple. It is understood that when a person throws a rock, approximately the entire brain is involved. The larger brain was developed to calculate trajectories and coordinate muscle and eye movement so rocks could be thrown with extreme accuracy to bonk rabbits. Standing on the hind legs also developed at the same time, also to facilitate rock-throwing leverage. Add the opposable thumb, which facilitates the split-finger fast-rock, and you have the story.
It makes perfect sense.