Blame Joey Porter.
After the Steelers' former linebacker and current assistant coach caused a hubbub during an injury timeout in Pittsburgh's playoff win over Cincinnati in January, the NFL felt the need to crack down on who can and can't be on the field.
Here's the entire listing from the league's 2016 Rules Changes and Points of Emphasis:
"Despite being a point of emphasis in previous seasons, the issue of coaches leaving the bench area to gain the attention of the game officials or entering the field of play for other reasons continues to be a widespread problem. The Committee sees no other recourse than to direct game officials to penalize coaches who do not comply with the rule.
A head coach can leave the bench area to get the attention of a game official when the snap takes place at a yard line not within the bench area and the coach is trying to call a team timeout or challenge an on-field ruling, and during an injury timeout to check the welfare of an injured player.
A coach cannot leave the bench area to question a game official, and at no time is a coach allowed onto the field of play. At no time can an assistant coach leave the bench area, even during breaks after scoring plays. These rules remain in effect during any timeout. Only incoming substitutes, team attendants or trainers seeing to the welfare of a player may enter the field. Violations will result in a 15-yard unsportsmanlike conduct penalty and the coach and/or the club may face additional discipline."
Even though players aren't mentioned specifically, they're not allowed to come onto the field whenever they want. So when Cam Newton crossed the line while celebrating a 95-yard interception return by linebacker A.J. Klein in Thursday's night preseason opener at Baltimore, the Panthers' quarterback became a perfect example. A 5-yard illegal substitution penalty took the touchdown off the board.
But wait, there's more.
The Panthers were flagged for unsportsmanlike conduct when an assistant went early in the 4th quarter when an assistant went on the field in the time between a Graham Gano field goal and kickoff.
Afterward, "learned" was a popular word in the Panthers' locker room.
Coach Ron Rivera:
"I think we got a little carried away with ourselves, and it cost us points. We can’t have that, especially if it was in the regular season and it really counts, ... "I understand the rule, and now it’s even clearer for me, as far as the timeout situation. We called the timeout and thinking we were allowed to [have an assistant coach on the field], but apparently not. It was a lesson learned. If you're going to do it, do it now."
Newton:
"I got too excited and the officials called me on it. I was hoping I would get away with just a warning, but we will learn from that and we won’t make that kind of mistake again, ... "I don't want to have a discussion about something that I've learned from. You live and you learn."
Klein:
"[Newton] came up and apologized, that could’ve possibly made the difference in the game. It’s just learning a lesson for the regular season."
S Kurt Coleman:
"I was like, 'This is really special. You guys really threw the flag on [Newton]?' I want to see it on film. I want to see how far he was on this field, ... "But you know what the great thing is? He’s actually engaged in the game. He was excited for the defense. He got the flag, but he’s actually in the game. That shows a lot about himself."