Kind of missing the point of people directly tying this last concussion to his history, as if it should have been a warning sign that he was going to get KICKED IN THE HEAD.
If John Doe has a history of knee injuries, and some random mugger happens to club him in the knee during a robbery, the response isn't "well we should have known he'd get his knee injured, he does have a history after all". That's my point. It was a fluke.
*edit*
Unless you mean to imply that the kick to his head was of a specific intensity, one that wouldn't injure someone without previous concussions, but would injure someone with a history of them? Cuz that's not really how concussions work.