... There is a fight playing out, the kind that starts from just a few people but erupts to everyone in the bar, and it cuts to two men looking at eachother very normally, but one then slaps the other, who slaps back, and they end up fighting too.

I think it happens in a stadium or a bar... It's very little to go on I know, but that bit really cracks me up, they have no reason to fight other than them being in the vicinity of a mass bar brawl.

I think it's from one of the earlier series. Thanks to anyone who knows which ep it is from!

I'm not sure if this is what you mean, but there's one similar random slapping scene in season 2, episode 6; The Mexican Staring Frog of Southern Sri Lanka.

Jesus has his little talk show and the boys and Jimbo start their bickering on the stage (and Cartman throws a chair at a catatonic Ned). The whole audience in the studio starts fighting after a random woman gets involved the fight. There are these two men in the audience who look at each other after which they began slapping one another. It doesn't start the fight, it's just part of it.

I hope this was useful.