As far as I can tell, Barr, then a private citizen, sent an unsolicited memo to people in the Justice Department that all but screamed "MEEE! PICK MEEEE, Teacher! I know this one!! No Obstruction!!!"
Barr authored memo last year ruling out obstruction of justice
In legal/investigatory terms, he has sinned: A prosecutor dismissing charges before any evidence has been presented. It makes him look like a chump. He's an embarrassment to his fellow lawyers. And he volunteered for this duty.
Some lawyers really are attached to the rule of law. That doesn't mean they're all honest, obviously, but you at least go through the motions. Barr cut through all the formalities, which is lethal because formalities are what the profession makes its living doing.
I don't know if I'm expressing myself well here, but the stupidity of this situation is that Trump didn't even have to ask. Barr should be about as welcome among lawyers right now as an amateur giving it away while honest sex workers across the street toil to earn a living.