I saw the replay.

He connected and snapped his head. Anytime your head gets accelerated that rapidly, even if the punch is glancing, its down time. The blood just stays put while your head leaves it behind (inertia) and you go down. Your brain temporarily is depleted of oxygen in some important regions. Even if it is just a short time. Then Jackson was on top of him pounding him. They dont want these guys to get hurt too bad. They need them to fight again so the refs jump in rapidly.

He torqued the crud out of his cranium. It happens in this sport. This is not boxing with padded gloves.