Why fire and not admin leave?

The investigation is just starting. He's been indicted but not proven guilty yet. What if the facts come to light and the issue is overblown or wrong?

He also hasn't had the chance to properly defend himself.

This is a permanent fix for a potentially temporary problem, and that is short sighted

I think it will come to light the board was aware well before it became public. There are likely more damning details that are yet to be public.