Anyone else find it weird that a knee to the hard shell and molded padding of a helmet would concuss an athlete?
It's pure physics, brother.
Thanks to that kid on Jerry McGuire, we know that the human head weighs a bit less than a small bowling ball--somewhere around 10 pounds. And it sits on a swivel mount called the neck. Even with a very muscular person, that swivel mount can't stop sudden-impact jarring. When the impact comes from the motion of another body that weighs 180-300 pounds, the 10-pound object is going to feel it a lot more than the other fellow. The head is going to move on its swivel, forcefully.
The hard shell and inner padding of the helmet help some. Particularly, the hard shell spreads the impact energy out, so it's not all concentrated at a single spot. That's good; we wouldn't want our players' skulls to be caved in at the point of impact. And the padding reduces some of the rapid acceleration, gives the head a smidgen of time to "get up to speed" rather than all at once. It's still not enough if the impact is forceful.
When another person collides with that head, whether with a knee or an elbow or their own head, there's a lot of energy being transferred. Newton's Third Law tells us: for every action, there is an equal and opposite reaction.
Chandler's head took all the energy of a moving player in one instant (okay, 1.3 instants, thanks to the helmet padding). It was jarred to the side. Sudden acceleration. That's what causes concussions, the brain being jarred around inside the skull.
Sure, the other player might also have banged up his knee a bit, but the inside of the knee isn't filled with soft, squishy goo that is easily jarred. So the head takes an inordinate amount of the damage.
It's just physics.