Is it okay to ask a former tenured professor why he suddenly left his job? He's also not listed in the faculty directory / contact list any longer.

I used to meet him about twice a semester to talk about life and academics, so he knew me fairly well.  

He was tenured and, to me, was brilliant.

Can I email him to say hello and ask why he left? Or is that being too nosy and inappropriate?
Unless you are a very good friend, you should not ask the question. You can, of course, continue to meet him in private if he is up for that. I do not recommend prying into the issue if he does not talk about that from his own initiative.