No idea, but given how sudden and out-of-the-blue this is, and the fact that he was co-founder of OpenAI (meaning that to some degree the board is pushing him out of his own company) you can make an educated guess that's it's huge.

~~My guess, personally, is financial malfeasance,~~ if only because personal misbehavior usually involves some hemming and hawing before a company concludes that the ejectee "is no longer a good fit and does not represent the values held by our organization." This was very sudden, no one saw it coming, and that's not too usual.

I guess we'll see, lol.

EDITED TO ADD I've changed my mind after a closer look at the wording of the board announcement. Given the language (Altman lied to them) and the chairman stepping down (but remaining in the C-suite) I’m starting to think that maybe Altman was trying to take something in an unapproved direction and present it as a fait accompli but got found out before he was ready to reveal it:

Mr. Altman’s departure follows a deliberative review process by the board, which concluded that he was not consistently candid in his communications with the board, hindering its ability to exercise its responsibilities. (paragraph 3)

As a part of this transition, Greg Brockman will be stepping down as chairman of the board and will remain in his role at the company, reporting to the CEO. (paragraph 6)