He wanted to shout to someone who was behind him. He didn't "turn around" first. He only turned his head, not his body. So he was shouting over his shoulder.
Similar to "he looked [back] over his shoulder." Of course there's another expression, "to look over someone's shoulder," which means to watch every move someone else is making while they're trying to work. "I don't like to have somebody looking over my shoulder." (It's used figuratively. They might be watching you via computer.)