“…and they withstood King Uzziah and said to him, "It is not for you, Uzziah, to burn incense to the LORD…” 2 Chronicles 26:18(a) (ESV).
Paul Manafort has filed a motion to dismiss the charges against him noting that Rod Rosenstein could not appoint Bob Mueller to any investigation outside the scope of the 2016 presidential campaign.
The legal reasoning here is that Jeff Sessions only recused himself from issues relating directly to the Presidential campaign.
This means Sessions retains the authority to direct and supervise any litigation related to non-collusion charges.
In short, it seems Mueller has overstepped his bounds.