How does a professor lose their tenure over a sex scandal which he was not involved in?
The Dean of the medical school is not being fired because he was involved in sexual crimes.

He is being fired because he poorly handled the Title IX investigation. He utterly failed to ensure the safety of student and patient safety, and he bears responsibility for that.

To characterize him as being "not involved in" the scandal is not accurate - the scandal is not only Nassar's actions, but that they were allowed to go on as long as they were without being addressed.