[SpikeGronim]

They say no evidence of illegality, but there is evidence of poor judgement. Without commenting on the specifics of this incident that's a valid reason to fire a CEO/co-founder/exec. I have no particular knowledge of what happened but if GitHub thinks he showed poor judgement then it is reasonable to ask him to resign.

[asuffield]

For example, if you didn't actually do anything legally wrong, but made it look an awful lot like you did by screwing up and being a dick about it, and you're a company executive, then you can resign or get fired.

I suspect that this is not a million miles from what happened here: not actual discrimination, but enough of a blunder to look like it.