Physicality and breakneck pace resulting in more knocks to the head.
NHL has got bigger, faster and crazier
There is a concussion crisis in ice hockey, and it is wiping out the best players in the game.
You know it is bad when it takes out Chris Pronger.
With the announcement this week that the rough-and-ready Philadelphia Flyers captain was done for the season because of headaches and dizziness, the big picture came into frightening focus.
Sidney Crosby, the Pittsburgh superstar, is back on the sidelines after a brief return. Pronger's teammate, Claude Giroux, who was leading the league in scoring, is out.
Ottawa's Milan Michalek, the NHL's top goal scorer, is out. And so are Carolina's Jeff Skinner, the reigning Rookie of the Year; Mike Richards, the former Flyers captain who was ripping it up in Los Angeles; Buffalo mighty mite Nathan Gerbe; and star defencemen Marc Staal, Kris Letang, Joni Pitkanen and Michalek's brother, Zbynek.
All with concussions.
Why are so many of the game's best players - and so many players, period - sustaining head injuries?
It is especially alarming when you consider that the NHL introduced a ban on head shots in the summer and had spread the message that transgressors would be penalised with long suspensions.
Among the factors to consider: the players are bigger and faster than ever, which inevitably means collisions are bigger and faster, too; the rules have played a part, as the NHL cracked down on obstruction after the 2004/05 lockout and re-introduced two-line passes, both of which increased the game's speed.
And there are a multitude of secondary contributing factors, such as coaches who demand that players finish their bodychecks with authority and a win-at-all-costs attitude. It is often a split-second decision that can be the difference between a blatant hit from behind into the boards and an innocent shoulder rub-out.
Finally, there's the fluke factor. Nobody has pinpointed the hit that ended Pronger's season. Crosby had some run-ins with Boston's David Krejci that might have led to the return of concussion symptoms, but he also ran awkwardly into a teammate. Giroux had fallen to the ice and his linemate, Wayne Simmonds, tried to jump over him - and ended up kneeing Giroux in the back of the head.
Milan Michalek ran into a teammate. It's a high-speed game and collisions can happen quickly and brutally, and sometimes between players on the same side.
There is nothing the NHL can do about that.