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?
He may have PTSD and he may have had 1,000 hours of firearms training, but if you empty your magazine the way he did, under the circumstances he did, you're incompetent to be a police officer. Period.
And even he apparently recognizes that since he resigned (though whether he'll just go get hired the next town over is probably a decent bet).