Why was Billy's head lopsided?

From the text:

The thing about Billy was that he had a lopsided head. Bertha Whitefoot, a half-­Indian woman who lived in a shack near the depot and kept about fifty dogs fenced in her yard, said it was because Billy's mom hadn't turned him over at all when he was a baby. He just lay there in the same position day in and day out, and the side of his head that was pressed against the mattress got a little flat.