The board may very well have met for this very reason, or perhaps it was at this meeting that the lack of candor was found or discussed, but to hold a board meeting there is overhead, and if the board is already in agreement at the meeting, they vote.
It only seems sudden to outsiders, and that suddenness does not mean a "night of the long knives".
MS owns a non controlling share of a business controlled by a nonprofit. MS should have prepared for the possibility that their interests aren’t adequately represented. I’m guessing Altman is very persuasive and they were in a rush to make a deal.
It’s questionable how much power Microsoft has as a shareholder. Obviously they have a staked interest in OpenAI. What is up in question is how much interest the new leaders have in Microsoft.
If I had a business relationship with OpenAI that didn’t align with their mission I would be very worried.