The board has given no real reasoning for why they fired him. Until they do, there's no reason anyone should consider this anything other than an internal power struggle that resulted in a coup.

And Sam didn't have a job anymore. Why shouldn't he go work for Microsoft? He was pushed out of OpenAI, is he contractually bound to never do something different?

I'm not the one contesting it, but there's a strong contingent of people who believe Altman's interest is in developing AGI and little else. To them, him taking that position could be explained him positioning himself to affect broader influence.

That's not my personal interpretation, but it is at least a little surprising that the rift is between him and his BOD. Presumably they would all have the same financial incentive to monetize their project, not just Altman.

Personally, I think people being quick to draw any conclusion from this are putting the cart before the horse. It's not clear to me at all what the competing interests are, if it's not just completely political posturing to begin with.