In my opinion this is most likely.
Scenario A, Sam Altman continues as CEO and the for profit arm of OpenAI continues to call the shots, growth and market share continue to be priority number one.
Scenario B, Sam Altman is fired, OpenAI non-profit board (re?)-asserts more control. Safety, alignment and other things like that take priority over growth and market share.
x% of Employees are Sam believers, and when the remaining x% of ambiguous/non-Sam believers realize the first x% might leave, their PSUs would be worth significantly less, so they sign the letter as well. There is also the peer pressure / fear of retribution factor as well once it becomes likely there is even a chance of Sam being reinstated.
Many employees choose scenario A because it is likely their "profit sharing units" will be worth more than with scenario B. There's a non-zero chance that OpenAI (the for profit arm) eventually joins the ranks of the "FAANG" companies. Those PSUs might be worth millions today, but in the future could be worth "fuck you" levels of money.