I wouldn't call it a fluke. Big Foot found his opening and capitalized. But no one in his right mind can say that Overeem didn't cost himself that fight by acting like an arrogant twit. He left his hands down. He left his chin out. And he hardly poured it on during the first two rounds. He was content to to stand there and smirk, and he only engaged when Silva came after him (and he won each of those exchanges in the first two rounds).
I don't normally give the loser the benefit of the doubt, but there's no question that Overeem is a superior fighter to Big Foot Silva. But, as others have said, it's HW. Cain Velasquez would have gone down to that barrage had he been stupid enough to play 'chicken' and allow Big Foot so many openings. Overeem could have lost to cans like Bob Sapp fighting like that. It's why you don't screw around at this weight class.