As you illustrate yourself, it's very hard to predict how a challenge for a high ball like that might end up. Which is why, when one CB move to challenge, the other moves to cover. That's basic defending. Had there actually been a sign that he was preparing himself for another outcome, or that he was covering for Shaw, it might have been different, but there wasn't. He was in no-man's-land, ball watching, only reacting when Ayew blitzed past him. You can list as many eventualities as you like, it's completely meaningless when it's obvious that none of them were on Harry's mind, and when only luck would have had him in the right position.