From: https://openai.com/our-structure

"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.