Yeah, it wasn't a big punch at all.  And his head hitting the canvas was the reason his body flexed from head to toe.  I think it was a combo of an underdeveloped chin, genetics (some are predisposed to having a chin), and some left over shock from the first knock out by Tarver.  He came back too quickly to fight after his first career knock out loss.
Nola, I too was a MMA guy.  I kick boxed amatuer for 6 years, and then got into Jujistu.  In MMA, I had 3 fights won 2 by submission, both knee bars, and one knock out.  I lost by heel hook.  Got out of it because, even when you win, you feel like you lost.  But I love it to this day.