Once the defender lowers his head he’s no longer playing it right. That’s the rule. It’s pretty black and white.
Again, that's trickier as a universal rule than it sounds.
It's trickier than avoiding the offensive player's head as a target.
I do wish there would be more candor about the truth of drawing these broad, bright-line rules to try and create a safe harbor and change the nature of the game (which is good! They should not be ashamed of that!) rather than making individualized determination of intent and risk in individual instances.
But if they apply the "you may not lower your head ever" rule evenly, that is going to mean a TON of throwing guys out of the game for no-intent, low-risk hits. So instead we have certain areas of the rulebook that are "open in case someone hurts a QB", and that's kinda annoying and doesn't serve player safety interests as well.
I'm not a general purpose "let em play" critic of this kind of thing, for whatever that's worth. I think the stuff about blows to the head against defenseless players is really working and changing the way defenders play for the better. You CAN tackle without headhunting.
It's just that the one last night seemed to be more clearly a coincidental accident initiated by the offensive player than these things usually are.