Straight from the boss's mouth:
http://www.buccaneers.com/av/flvideo.aspx?id=1576
Basically, Coach Gruden felt that it was in the best interest of the team to start Gradkowski because he liked the way he moved the ball, but he's struggling a lot lately. He said that he contemplated starting Tim several times in the past. He can't dwell on the past. Time to move forward with Tim.
Good luck, Timmy!