Why did Mel Gibson start his anti-semitic tirade? Did something trigger it? It seems a strange thing to do.

One of the effects that being drunk has on some people is that they lose their inhibitions. In other words, they don't self-censor the expression of thoughts that, when sober, they know would get them into trouble.

Mel Gibson's father, is a religious zealot and Holocaust denier. I don't think it's unusual for the son a of a Holocaust denier to turn out to be an anti-Semite.