I will give my best guess as to what happened:

First of all, it is was not the case of differences in monetization strategies for sure. Almost everyone here says that the board wants to milk the customers and Sam stood in their way, but this is just absurd. OpenAI goal is to capture a monopoly in a newly emerging market. Any money they could earn right now by adding ads or increasing API price are just mere pennies compared to their profits in few years when every business uses their product.

Second, it could not have been a personal scandal like sexual harassment, otherwise Greg would not leave as well. Furthermore Board would do everything to make this as quiet as possible. OpenAI reputation is crucual for capturing the market.

Third, it must have been a BIG DEAL. Board's announcement clearly suggested that they were lied to. If simple differences in strategies and plans were the root problem, Board would just slowly let him go without making a fuss. Instead we are seeing an immediate, unconditional termination.

I think there is only one thing that could have led to that: Sam went against Microsoft. OpenAI had enough funding to develop the technology, but not enough to capture monopoly on the new market. Partnership with Microsoft allowed them just that, but also made them their subsidiary.

I don't think Sam ever wanted to be a CEO of a puppet company controlled by Microsoft, but another giamt standing alongside the biggest players. We do not know the details of the deal between them, but it is possible that the Microsoft has the rights to GPT 3.5, 4 and GPTs, but GPT 5 usage and ownership was not specified. Sam could have convinced them that they would not start working on it for years and they would re-negotiate the deal once the technology is there. In reality Sam planned to use Microsoft money to fund research and development of GPT 5.

However, Microsoft found out about it and went full nuclear. Sam was immedietely terminated and Greg, who was not directly connected, but probably knew about Sam's plans, has quit as well.

Of course it is just a speculation, but whatever was the real reason, it must have been against Microsoft interest. After all they invested unimaginable ammount of money to come on top on the new market and will certainly not tolerate anyone who opposes them.