From the press dispatch:
"The additions of Rashard Lewis and Mo Evans combined with Josh Howard’s return from injury gave us a logjam at the small forward spot," said Grunfeld. "Coming to terms on a contract buyout with Al was mutually beneficial, as it allows us to clear space in the rotation and allows him the chance to pursue an opportunity with another team."
Thornton was originally acquired from the L.A. Clippers in a three-team, six-player deal on February 17, 2010. He appeared 49 games for the Wizards this season, averaging 8.0 points and 3.2 rebounds.
I liked Al as an interview but he was pretty much a square peg in a round hole all season. He didn't have enough of a jumpshot to make sense in Flip's system and basically his performance crashed after the abdominal injury.