My guess is that Clark angered Tony Blair, who then prevailed upon Clinton to give him the boot.
Recall that, after he told Clark "I won't start WW III for you", Brigadier Jackson reported Clark's order to eject the Russians from the Pristina airfield to the Imperial General Staff. Reading between the lines, the UK papers suggested that the PM was sorely pissed at the irresponsibility Clark had displayed. Especially since British troops were being ordered to do the deed.
In other words, Tony got Willie to do what Willie should've done in the first place, without being asked.