I think it took him a few rounds to figure out his opponent, then perhaps one or two more to formulate a plan and well...a sweet KO in the end. If I see a champ take out a challanger very early into the fight (1-4 rounds) I usually think of one of either three things has occured:

1) He caught him cold
2) He's an all time great that should be lauded through the ages
3) He fought someone who had no business being in the ring with the champ.