I assume it's just to prevent all the excess stuff you see in the NFL. The term we often here is to allow "spontaneous immediate acts of exhuberance by teenagers". I know many players are in their 20s, but you get the idea. High fives, chest bumps, etc. are fine. Quick actions that are borderline are often talk tos (i.e. a quick first down signal when they stand up is warned the first time but flagged if done multiple times or extended). Running to the other side of the end zone to do it only drew more attention to it.
Here is the exact wording of the rule (9-2-1a). (h) is the one that applies here. I guess if he does it after a bad play it's OK?
Specifically prohibited acts and conduct include:
1. No player, substitute, coach or other person subject to the rules shall use abusive, threatening or obscene language or gestures, or engage in such acts that provoke ill will or are demeaning to an opponent, to game officials or to the image of the game, including but not limited to:
(a) Pointing the finger(s), hand(s), arm(s) or ball at an opponent, or imitating the slashing of the throat.
(b) Taunting, baiting or ridiculing an opponent verbally.
(c) Inciting an opponent or spectators in any other way, such as simulating the firing of a weapon or placing a hand by the ear to request recognition.
(d) Any delayed, excessive, prolonged or choreographed act by which a player (or players) attempts to focus attention upon himself (or themselves).
(e) An unopposed ball carrier obviously altering stride as he approaches the opponent’s goal line or diving into the end zone.
(f ) A player removing his helmet after the ball is dead and before he is in the team area (Exceptions: Team, media or injury timeouts; equipment adjustment; through play; between periods; and during a measurement for a first down).
(g) Punching one’s own chest or crossing one’s arms in front of the chest while standing over a prone player.
(h) Going into the stands to interact with spectators, or bowing at the waist after a good play.
(i) Intentionally removing the helmet while the ball is alive.
(j) Dead-ball contact fouls such as pushing, shoving, striking, etc. that occur clearly after the ball is dead and that are not part of the game action. (A.R. 9-2-1-X)
(k) After the ball is dead, using forcible contact to push or pull an opponent off the pile. (A.R. 9-2-1-XI)