Theory:
Trump didn't trust WH Counsel McGahn and tested him by instructing him to fire Mueller. If he did the deed, he'd just undo it. There's no reason Trump wouldn't do it himself, since the media will still say Trump did it anyway so why not just do it himself, makes no sense. So McGahn doesn't do it, and quits instead? No. McGahn didn't do it, and Trump forced him to resign is the real story. McGahn was a snake in the field.