Underestimating costs could be the reasoning if Altman knew the costs would be higher then estimated, and didn't tell the board for an unreasonable amount of time. Burning through a ton of cash for months and not telling the board about it could be enough grounds for this sudden firing.
Of course we have no clue if that's what actually happened. Any conclusions made at this point are complete speculation, and we can't make any conclusions more specific then "this is probably bad news."
That only makes sense if Altman is the only one with access to the company's financials which obviously can't be the case. No one else noticed truckloads of cash getting flushed down the toilet?
It's certainly possible. Company financials can get very complicated very quickly, and it's possible that Altman was the only person (or one of a small number of people) who had the complete picture.
To be clear, this is only one possible explanation for Altman's firing. And for my money, I don't even think it's the most likely explanation. But right now, those who rely on OpenAI products should prepare for the worst, and this is one of the most existentially threatening possibilities.
Of course we have no clue if that's what actually happened. Any conclusions made at this point are complete speculation, and we can't make any conclusions more specific then "this is probably bad news."