It's pro forma for the ambassadors from the outgoing administration to proffer their resignation effective noon on Inauguration Day. The incoming president then says, no, no, please stay on until we have your replacement set up.

A transition team who knows these things would have done that - the Trump team, though, since he was not actually expecting a transition, had no idea how to run one. And accepted all the resignations.

This is the same reason the commander of the D.C. National Guard was dismissed at noon on the same day.

Never attribute to malice that which can be explained by incompetence.