There are many possibilities (and I can't really tell from skimming this writeup), but the possibilities absolutely include someone thinking "We got what we nee

There are many possibilities (and I can't really tell from skimming this writeup), but the possibilities absolutely include someone thinking "We got what we needed, now let's drop this person, and free the salary and bonus obligations". Or they might've decided they didn't want that employee for some reason, maybe even dudebro culture fit mismatch when he showed up looking fatigued, and were "sux2bu" about how they got rid of the person, firing fast. Or that project got scrapped. Or some other person had a look at the code and decided they didn't like it (or didn't like the partner whose pet project it was). Etc. A factor to keep in mind is that many, many shops nowadays really don't know how to build software that works sufficiently well, or they have low standards for what "sufficiently" means. For example, a few years ago, if your experience was only when the top priority was the appearance of growth, and then the next funding rounds and then exit, and there were all sorts of memes about moving fast and breaking things, etc., and you have no idea how to build systems that work, even if you wanted to... you might well think that developers are disposable and interchangeable commodities. (Someone else here noted the practice of a startup using offshoring contractors for key tech early on, which I think is strong evidence of this kind of thinking.)