Why didn't he throw over then? In short, an abundance of caution. There were zero outs, he's already moving towards first, he's left handed so he has to wheel around completely one way or another, basically the same reason every 10 year old is trained to go to 1st in that situation. He was moving towards third. ... He was in no way moving towards third base. The opposite, because he was moving(stumbling, as Tryptamine notes) towards first. How do you think that's even possible, for a lefty pitcher to field a ball on the third base side of the mound while moving towards first?