Why did TR knight leave Anatomy?

There is no actor named TR Knight. Are you perhaps talking about Isaiah Washington, who played Dr. Preston Burke in Grey's Anatomy? If so, Washington's departure from the show was due to a controversial comment he made during an on-set argument with Patrick Dempsey. Additionally, T.R. Knight, not I. Washington, left after playing Dr. George O'Malley because Grey's Anatomy killed off Dr. O'Malley.