Its either an illegal shift or illegal motion. In my opinion, both were violated, but having been on the sidelines for many, many games, illegal shift is called significantly more often than illegal motion.
Violating 7-2-6 is illegal shift
After the huddle or shift, all players of A must come to an absolute stop and shall remain stationary simultaneously without movement of hands, feet, head or body for at least one second before the snap.
Violating Rule 7-2-7 is illegal motion
Only one A player may be in motion at the snap and then only if such motion is not toward his opponent’s goal line. Except for the player “under the snapper,” as outlined in Article 3, the player in motion shall be at least 5 yards behind his line of scrimmage at the snap if he started from any position not clearly behind the line and did not establish himself as a back by stopping for at least one full second while no part of his body is breaking the vertical plane through the waistline of his nearest teammate who is on the line of scrimmage.