It seemed like a classic case of prisoner's dilemma, so (5) and (7). The more of your company that signs the petition, the lower the value of your PPUs, making it more attractive to sign. It reached a point where they felt OpenAI's value and their PPUs went to nothing if a critical mass joined Microsoft. In fact, if MS was willing to match compensation, everyone "cooperating" by not signing the petition is a worse outcome for everyone than just joining MS because they had already seen other players move first (Altman, Brockman, other resignations) - that is if we look purely at compensation (not even taking into account the possibility that PPU-equivalent at MS would not be profit capped). In textbook prisoner's dilemma, cooperation leads to the best overall outcome for everyone, yet the best move is to defect if you are unable to coordinate, which is not really the case here.
Further, even if an OAI employee did not care about PPUs at all, and all they care about is the non-profit mission of AI for the betterment of all humanity, they might have felt there was a greater likelihood of achieving that mission at Microsoft than the empty shell of OAI (the safety teams for example - might as well do you best to help safety at the new "leading" organisation, and get paid too).
The way I see it, Altman likely was not giving great board updates and may have been ignoring what they told him to do for months. Board removal was not a credible threat then, adding more board members would not do anything to change him.
The safety/risk calculus changed for him. It was now a decision between Sam and 700 employees working unshackled for MSFT or bringing Sam back to OpenAI with a new board, the former being way more risky for humanity as a whole in his mind.
Networking, Relationship building, both professional and personal, I'm sure there are overlaps. And echoing another request: Sales