Because it's what he does after the 2 feet come down. Among the items considered a football move is a 3rd step (while running, theoretically). In the video, Waller never got his 3rd foot down, right?
RULE 8 FORWARD PASS, BACKWARD PASS, FUMBLE
ARTICLE 3. COMPLETED OR INTERCEPTED PASS. A player who makes a catch may advance the ball. A forward pass is
complete (by the offense) or intercepted (by the defense) in the field of play, at the sideline, or in the end zone if a player, who is
inbounds:
(a) secures control of the ball in his hands or arms prior to the ball touching the ground; and
(b) touches the ground inbounds with both feet or with any part of his body other than his hands; and
(c) after (a) and (b) have been fulfilled, performs any act common to the game (e.g., tuck the ball away, extend it forward, take
an additional step, turn upfield, or avoid or ward off an opponent), or he maintains control of the ball long enough to do so.