Clearly, the Russian Command ordered withdrawal from around Kiev and Chernihiv, but why?
Possibly, they just assessed their losses and advances, revised their war aims to current conditions, and are reprioritizing the Donbas as their main effort. That is the generally accepted analysis - Russian doctrine is to reinforce success, and the Donbas is their most valuable goal, that is assessed as still achievable.
But another theory is that Belarus wants out of its role in the war. Those forces that entered from Belarus are the only ones withdrawing, and are doing it massively (although in an ordered fashion, covered by artillery and air). Today was the first day in weeks that no missiles or air strikes were launched into the Ukraine from Belarus.
It might be the sanctions, it might be the domestic political opposition in Belarus, it might be an updated assessment of the military situation, or the combination. Lukashenko has been running Belarus for over 30 years, and his first priority is likely Lukashenko.
[It might be the sanctions, it might be the domestic political opposition in Belarus, it might be an updated assessment of the military situation, or the combination. Lukashenko has been running Belarus for over 30 years, and his first priority is likely Lukashenko.]