slapping

It was only a joke, but it was enough to upset Will Smith enough to slap a man. What would someone have to say to make an Idahoan make enough to slap them?

A few weeks ago I was at a boys basketball game and I noticed that almost ever boy was slapping each other on the rear end as a way to say, "good job" or "go get em". I have always thought this was a little weird but it seems to be the normal thing to do so maybe I am the weird one??