Of course it does happen, but it's a lot more likely in MMA because you aren't given a chance to comeback. In football, if the weaker team manages to score a couple of TDs, the other team can still regroup and win. There's nothing that can change the game instantly because of one action. You can't regroup from being KOed lol or caught in a triangle. Give me an example of Sonnen/Silva, or Lewis/Volkov in any other sport, where a team was leading by a huge margin and then lost in the last bit of the game. One home run will never trump 10 runs. However one KO will trump 5 rounds of domination. The closest you get is other combat sports with KOs. But even there if you get KD, the ref steps in and gives you another couple of chances at least.