tldr: Sama stepped down,

No, he was fired.

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. The board no longer has confidence in his ability to continue leading OpenAI.

That’s as close to “he lied to us so we fired his ass” as you’re ever going to hear from a corporate board.