It's a bit vague as to why but they produced a document

>The Altman document consisted of dozens of examples of his alleged lies and other toxic behavior, largely backed up by screenshots from Murati’s Slack channel. In one of them, Altman had told Murati that the company’s legal department had said that GPT-4 Turbo didn’t need to go through the joint safety board review. When Murati checked with the company’s top lawyer, he said he had not said that.

I guess you can fire someone for lying and toxic behaviour?

It seems to have come to a head a bit over board member Helen Toner criticizing OpenAI's approach to safety. Altman I think wanted her gone and she wanted him gone hence the battle. I guess the employees and investors figured they wanted to be rich and could put up with the lying and toxic behaviour.

One of the few powers of the board is to fire the CEO. I do not see why there is any need for codified reasons. If the board thinks the CEO should do better, that is all that matters.

The CEO can be drunk, liar, cheat, steal, and be incompetent only as long as the board thinks it is ok.