1) A long standing disagreement between AI-safety and AI-profiteering with Ilya on one side and Altman on the other. Ilya (board member) was the one who told Altman to attend video call, then told him he was fired.
2) Some side dealing from Altman - raising new VC funds - maybe in conflict of interest with OpenAI, that was the final straw.
There also appears to be a lot of rumors about Altman's personal conduct, but even if true that doesn't seem to jibe with the official statement over the reason for his firing, or Brockman and others resigning in unison - more reflection of the internal rift.