Defensive holding is anything to slow the route down pre catch (Jersey pulling, hip hugging) and it has to be well before the ball is close to being cached. This is why you always see it when the ball hits the ground and there isn't a WR in the frame. With routs there are three phases. Within 5 yards, the route, the catch. This penalty happened post rout when the ball was already in flight and the WR was extending his arm for the catch (The ball was within the catch radius). At this 3rd phase of the play any contact that isn't a true play on the ball (DB's head turned and trying to INT) is going to get PI called. Like I said, the ONLY thing that would have negated the PI was if the corner had better inside leverage and used his contact trying to make an interception. The refs can say at that point the DB is trying for the ball not disrupting the catch. If you can't go for the INT (like in this case) let the ball go into the WRs hands and THEN break up the catch. Refs will also consider that legal. But, what you can't do is target the WR from behind from making the catch. The picture I showed earlier clearly shows Mooney was prohibited prior to the ball arriving.