I think I know.
The OIG report will show the Mueller investigation was itself a criminal enterprise.
The OIG can't release before the Mueller report is released, because Mueller the dirty criminal cop would just make up a bunch of bullshit to cover up as much of the dirt that the OIG report will reveal.
So Barr is forcing Mueller to wind down and release first, so that Mueller can give himself all the nooses he could possibly need to hang himself, which the OIG report will show.