Quote: Mr. Zucchini Head "It would have been difficult to ban him based on the video that was available. Unless there is another angle which I haven't seen. You can't see for sure whether his knee contacts the head and if it does it seems unlikely that it was intentional. There are a couple of stills which show Joel in a pretty vulnerable looking position with his head bowed low as he tries to get up. I wondered if the weight of the challenge forced his head into the ground and that was what caused the concussion. I'm speculating of course.'"
I think thats a pretty good assessment of how it happened,joel was in an awkward position and the pressure of austins tackle pushed his head down.