Ref has to make the call:
(f) If a player uses any part of his helmet (including the top/crown and forehead/”hairline” parts) or facemask to butt, spear, or ram an opponent violently or unnecessarily. Although such violent or unnecessary use of the helmet and facemask is impermissible against any opponent, game officials will give special attention in administering this rule to protecting those players who are in virtually defenseless postures, including but not limited to:
(1) Forcibly hitting the defenseless player’s head, neck, or face with the helmet or facemask, regardless of whether the defensive player also uses his arms to tackle the defenseless player by encircling or grasping him; or
(2) Lowering the head and violently or unnecessarily making forcible contact with the “hairline” or forehead part of the helmet against any part of the defenseless player’s body; or
The second one is probably the closest as to why it was flagged.