The fact that it annihilated their investment comes to mind.

The only defense the board would have is that they absolutely needed to fire the CEO to protect their precious non-profit charter. But for that to be the case they would need to show their homework. I don't see that happening and so they may well be exposed. At least two of the remaining board members are not positioned at all to take on Microsoft in a fight like that, they'll fold like chopsticks. Microsoft can keep this up for a decade if they want and not lose money on it.

I assume Microsoft will countersue with their own interpretation of the contract. However baloney that is, they only have to drag the case along until the former shell of OpenAI runs out of money and gives up the ghost. They could spend $10Bn on this and still come out ahead compared to buying OpenAI at market rate.