I met Kimbo in person, and he's one hell of a beefy guy... Hard to imagine he'd really go down with one quick punch like that, but then again it was pretty fast and he was moving/leaning forward, so it is possible that he was stunned and stumbled forward.
I just don't think an obvious setup win makes any sense in this case, it is more than likely a combination of good luck on Petruzelli's part and bad luck on Kimbos.