So this is the non profit board. Maybe he was trying to influence the board members to make OpenAI a for profit company and the board saw that as a traitor move.

The only other business thing I could think of is he moving openAI's money to one of his other venture or for some other personal gain.

I don't think spending too much money in openAI could get him fired like this in current environment. Neither does overpromising stuff or something.