tl;dr: professor tweets statement university disagrees with and gets fired, professor writes open letter rebuking them
they weren't fired. They survived an attempt at being fired. Instead, they resigned because in their opinion, the university made continued employment impossible. University leaders are a in a tough position today of having to balance open-minded, thoughtful professors who make unapproved external statments to the press, with a raging mob that wants to cancel people that don't say things they like. I imagine the dean/VP of the school probably internally supports Shapiro but has to make it look like they don't.