My only explanation is that Bourque and Diaz were forced to fight each other until they both gave each other concussions. In the process they suffered memory loss, which is why neither of them has any idea how it happened.
This is more typical of how concussions occur in hockey. Any hard landing to the head could result in a concussion. Whether there's fighting or not is largely irrelevant to fixing the concussion issue. Probably, the most easiest option is to slow the game down and not call as much interference and holding penalties.