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.