Earlier quoted context omitted.
His writeup: https://zachholman.com/talk/firing-people We cannot know for sure whether it has anything to do with his firing (and he doesn't mention it in the writeup), but Julie Ann Horvath did accuse Zach of harassing her at GitHub.
The sibling to this comment links to an article about that. This is the first I've heard of the situation, but that article doesn't say anything about her accusing him of harassment. Rather, it says that the two of them were in a relationship at the time, and she accused him of being complicit by sharing information with her harassers and otherwise ignoring the issue. Regardless of what happened or didn't happen, his…
"complicit in the actions of both Tom and Theresa Preston-Werner and even admitted to plotting with Theresa Preston-Werner to get women at the company fired. He should be let go from GitHub and I regret being kind to him in previous interviews"
Again, unsubstantiated, but if for example he was just mean but not harassing, that could explain the firing (while Github claiming no harassment occurred).