Rule 9-3-5 specifies when a block in the back is legal and illegal. Exception 1 states that offensive players who are on the line of scrimmage at the snap within the blocking zone may legally block in the back in the blocking zone subject to certain restrictions.
A. A player on the line of scrimmage within this zone may not leave the zone, return, and then block from behind.
b. The blocking zone disintegrates when the ball leaves the zone.
So question 1 becomes what is the blocking zone? The blocking zone is a rectangle centered on the middle lineman of the offensive formation and extends 5 yards laterally and 3 yards longitudinally in each direction.
From the single vantage point in the video, it appears that A72 is an offensive player on the line of scrimmage at the snap who stays in the zone.
HOWEVER….
Question 2 is, Had the ball left the zone at the time of the block in the back? It certainly appears that the quarterback had thrown the ball downfield at the time of that block so the rule could warrant a block in the back foul as the zone had disintegrated due to the ball leaving the zone.
So does this warrant a flag? I could support an official making a call on this either way. By rule you could flag this action. I could even see an official issuing an unnecessary roughness foul if this player had repeatedly ignored warnings about such behavior. That action was away from the play and not at the point of attack. Most coaches would rather address A72 on the sideline and prevent any further escalation during the game.