Jon mentions in a blog post that "leadership within the company neither understood community management nor was willing to learn", and that his decision to leave was connected to the incident with firing Monica.
Also reported on The Register: "Stack Overflow makes peace with ousted moderator, wants to start New Year with 2020 vision on codes of conduct"
We’re sorry to see you go.
We understand there are some folks upset about the decision made this week. We aren’t going to share specifics out of respect for all individuals involved but this is a site reaching millions of people and we have to do what we believe fosters a spirit of inclusion and respect. When a moderator violates that, we will always do our best to resolve it with them privately. When we can’t we must take action. This is always done based on what we believe is best for all SE users.
Also reported on The Register: "The mod firing squad: Stack Exchange embroiled in 'he said, she said, they said' row"