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)

Whatever it was it's spicy enough that they're trying to bury the press release in the late Friday afternoon news graveyard

Edit: even better, whatever they're trying to distance themselves from is so important they didn't even wait for the closing bell on the market.

I interpret their wording that he lied to the board repeadetly and made some unethical backdoor deals of evil.

Makes me wonder if he wanted to put ethical guardrails on the product that would’ve been less profitable for the board.

Unverified, but I saw some rumor about sexual abuse in his past. Look it up.