Below is a screen grab of what actually happened on the play. There were two issues from an execution standpoint. First, Lane Johnson doubled the defender already being blocked by Evan Mathis, when it appears that his responsibility was probably to block Tamba Hali (circled). Hali was instead completely unblocked with a free run at Ertz. Secondly, the snap by Jon Dorenbos was a little behind Ertz. Ertz had to take a step back to go get it (as you can see in the shot below), which slowed him down in attacking the end zone: