UPDATE: OK, here's his explanation:
"I'm a performer. I push the envelope. I work in a very uncontrolled manner on stage. I do a lot of free association — it's spontaneous, I go into character. I don't know. In view of the situation and the act going the way it was going, I don't know. The rage did go all over the place it went to everybody in the room." Richards seemed baffled by his own reaction on stage. "I'm not a racist, that's what's so insane about this," he said.
I don't find that completely convincing, although I can see that's it's a possible explanation. He was trying to go into a character, and chose a completely inappropriate one. He was too deep into the character to realize what he was doing as he was doing it.