I'm not sure I should admit this, but I was fired from a job (while I was in college) in rather similar fashion. I don't think it's even that unusual.

Think about it. The boss KNOWS this guy is making fun of him in a very public way. He's calmed down (he thinks), he's going to have a really rational conversation with this guy. He's going to work with him... and then the guy starts smirking a bit, starts sort of mocking him a bit. The real anger comes out.. he gets pissed... and great drama ensues.

It happens.