It happened because McGirt had no respect for Davis whatsoever.

Buddy's left was low and he was telegraphing it - Davis really needed to throw some right hands over the top of Buddy's left to establish himself and make Buddy respect him (the jab was pretty much all he threw all round)

Sometimes boxing is like poker, you have to bluff and make out you have a better hand than you really do. Ali was a master of the bluff. Davis was never a hard puncher, but he should have set down on at least a couple of right hands to make Buddy ease up with the charging. Then he would have found his gameplan (which, strategically to me, seemed like he didn't really have one other than move, move, move) a little easier to execute.

But yes, I agree with Chris - Howard was so worried about the hook he didn't even see the right hand coming. His left hand should have been tighter to his chin and he seriously miscalculated his foot speed while attempting to slide up the ropes.