"Second, because the board is still the board of a Nonprofit, each director must perform their fiduciary duties in furtherance of its mission—safe AGI that is broadly beneficial. While the for-profit subsidiary is permitted to make and distribute profit, it is subject to this mission. The Nonprofit’s principal beneficiary is humanity, not OpenAI investors."
So, if I were to speculate, it was because they were at odds over profit/non-profit nature of the future of OpenAI.
The fact they timed the announcement actually implies some planning, which means the reason couldn't be so damaging that they had to fire him immediately after discovering it. (Of course, it's possible that only by coincidence, an unplanned revelation happened at a convenient time to fire him.)