Board Thread:General Discussion/@comment-4310945-20131108013557/@comment-5398287-20131108021610

We don't know for certain. However, there is some evidence pointing to Uryu only faking his betrayal: Yhwach acknowledged such a possibility and set up the announcement of Uryu as his successor so Uryu would be forced to follow his orders, regardless of whether or not he intends to betray Yhwach, because the Sternritter are intently focused upon his actions and are watching his every move. Additionally, though we don't know for certain, it's likely that Ryuken has told Uryu how his mother died - and who killed her. But, for all we know, Uryu has found a reason to join Yhwach that is important enough to him to supersede both his mother's death and his friendship with Ichigo, Sado, and Orihime. I have no idea what such a reason could be, but it's a possibility.