Looking closer, Sexton was not even supposed to guard Young, it had to be Cody but Sexton was ball watching and completely missed streaking Krejci. Cody even points out Kreijci to him but because Sexton is a defensive idiot he does nothing and Cody is forced to run to cover Krejci himself, leaving Sexton on Young for his final defensive backpedaling effort. Sexton managed to make three defensive mistakes in the span of two seconds, which is, honestly, quite impressive.
Also, kudos to Cody: he can become a very good defender one day.