I can't think of any specific "exile event", but it did seem some time before he left that there was a trend in the Ruby/Rails community that made him feel unwelcome. That trend was "replace why's libraries and make noise about how his library wasn't any good".
Hpricot in particular was under heavy assault. I'll be the first to say that maybe it wasn't always the most performant solution, but for a long time it was the only good solution (and I tried a few). So when people are ripping your libraries left and right, forgetting to pay a little bit of respect to how big an impact he had on the community, then yeah...I could see how he'd feel the need for a self-imposed exile.
While I've never followed either of the stories that closely, isn't that fairly similar to what happened with Zed Shaw? Only in Zed's case he ended up finding a happy home in the Python community vs. completely disappearing from the face of the earth forever.
Zed (his real name? Seems like one) became famous by putting on a loud obnoxious persona, plus complaining that people used his work (mongrel) without appreciating him.
Why (not his real name!) became famous for his work's weirdness. And his "enemies" disliked how capricious and weird his work was, not the sort of thing to build a Real Serious Business on. and he fled from the public eye into self-imposed exile when people started piercing his intentional veil of anonymity.