I just watched "Ice Guardians," a documentary about enforcers in hockey. They claimed that only a small percentage (can't remember off the top of my head, maybe 5%) of concussions in hockey were due to fights. The majority come from the high speed nature of the game, especially after 2005. The game became a lot faster then. Big, slow players were replaced with smaller, speedier guys. The trapezoid was introduced which results in tons of defenders getting plastered behind the net while trying to retrieve the puck.