Richie McCaw is always in the thick of the action, making a nuisance of himself at the breakdown. He's so good at turning the ball over, and disrupting the opposition by playing on the edge of the law - that the opposing team sometimes get frustrated.
This time, McCaw gets caught on the wrong side of the ruck. It's not clear whether David Pocock is trying to regain his balance, or whether he knew McCaw was there - but there's certainly contact from Pocock's boot with McCaw's head.