Werdum's exit comes on the heels of a failed contract renegotiation following the loss to dos Santos. The Brazilian came into the fight with the expectation that he would soon be contending for a heavyweight title shot.
The knockout at UFC 90 derailed anything of the like in the near future, and spurred a renegotiation of his contract. Werdum and the UFC were apparently unable to come to an agreement. He has already opened talks with other promotions in the U.S. and Japan.
That's a stunning fall from grace for a fighter who once thought that he would get the first title shot against the winner of the current four-man UFC heavyweight tournament. But it's not altogether unsurprising: Werdum was never a huge draw, and that loss to Cigano was devastating. And, as Zach Arnold notes, when it comes to fighter contracts, UFC has total control. In this case, they're using their control to say goodbye to Werdum.