From what I've read, and based on the fact that OpenAI was NOT organized as a corporation, but a non-profit, I'd guess that Altman's gung-ho attempts to open the throttle on monetizing the corporate subsidy created to be able to make profits were in contrast to OpenAI's actual mission - which was to create an AGI (Artificial General Intelligence) that would think and reason as a human would, only "better" - is what caused the board to fire him.
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My guess is the "loss of confidence in his communications with the board" were because he didn't make it enough clear that the primary nature of partnering with Microsoft was to enrich investors, rather than simply fund the AIG development.