The hand of Mandamus!

Sounds like a bad horror movie.  Legal Insurrection (of course): "Appeals Court Orders Judge Emmet Sullivan To Explain His Actions In Michael Flynn Case."

The D.C. Circuit has issued an order which is as unusual as Sullivan’s own conduct.

I'm not a legal expert, but I don't get it either.  What's the end game here?  It's not like the judge can appoint somebody to prosecute the case against Flynn.