Self preservation would be one motivation. .

The 2019 RAND report gameplan for this conflict makes it clear that, if the worst case scenario happens and Russia gets stronger over the course of the conflict rather than being depleted as intended, an offramp should be found immediately to minimize Russian gains. Ukraine is losing the war, and supplies are now limited to a dripfeed. Their suzerain is stating pretty clearly that they expect compliance. -- Logically, in line with US think-tanks, the next step is to assassinate Zelensky and find someone more conciliatory.