I can think of four possible causes, none of them encouraging:
1. Khalilzad has given up on Iraq as a lost cause.
2. BushCo has decided to get rid of him in order to implement a change in policy (e.g., backing a coup that would produce an unparliamentary, strongman-led “government of national salvation” or, alternatively, backing partition).
3. BushCo has decided on a change of policy (a coup or partition) that Khalilzad wanted no part of.
4. Prime Minister al-Maliki is tired of Khalilzad’s demands that the Iraqi Security Forces be purged of, and ordered to fight, the partisan/communal militias, and either told Khalilzad he was no longer welcome or sent word to Washington that he wanted a new ambassador.
Only one thing is certain: If Khalilzad still thought his strategy had a chance of success, he wouldn’t be talking about leaving.