Did someone upset him, and he defected?
Perhaps he merely found more like-minded souls at the Codex and found more conversations and topics to his liking? There are numerous possibilities.
I haven't bought this game yet - although I did very much enjoy the original Planescape: Torment back in the day. I'll no doubt return to this review when I inevitably do pick it up and play which won't be for awhile - as I don't generally play games upon immediate release. That said, I'm naturally looking forward to Corwin's review too.