Investors are highly pissed off that Altman was let go.
It's a struggle between two different ideologies.
OpenAI was created as a non-profit organization, hence the word "open", so they technically have no owners and have no shareholders. It is run by an indepedent board (who have no ownership stake) like most non-profit organizations.
The problem is that because they wanted to generate (receive a lot of) investments, they created a wholly-owned for-profit subsidiary called OpenAI Global, LLC.
The non-profit is not meant to make money, but the for-profit subsidiary's goal is definitely to make money.
From what has come out about the whole situation, the non-profit board fired the CEO because he was "not being candid" (their words), but speculation has been that the CEO was more focused on the for-profit company, which conflicted with the mission of the parent, OpenAI, company.
However, the way they went about firing the CEO with no warning and without any specific event or actions by the CEO caused the internet, and subsequently the tech industry and the media, to react very strongly. They also excluded the Board Chairman (the President of the company) from the meeting and the vote.
It also came out that the board did not tell Microsoft, who has invested supposedly over $10 billion in OpenAI (no idea how that is structured, what they get, etc.), about it ahead of time, and rumors are spreading that Microsoft was not happy with the decision and not happy, given their huge investment, they were not consulted.
The rumors now are that the OpenAI non-profit board is expected to resign if the CEO and President return.
In the meantime, 3 OpenAI researchers also quit. I have no idea of their roles or whether it is because they fired the CEO or because there is speculation they are bringing him back.
Also, the fired CEO and President are already working on a new AI venture and supposedly have a lot of companies and investors lining up to fund them.
Regardless, the core of this situation is because the non-profit and for-profit ideologies conflict with each other, especially because it is AI and there is a lot of excitement and concern about its impact on the world.