Mueller realizes that his entire
raison d'être is tied to demonstrating Russian interference in the election process and possible collusion with the Trump campaign. The recent revelations about the Steele dossier, Uranium One, and the cooperation of the DOJ and FBI in providing Congress with documents along with the beginning of new Congressional investigations have forced Mueller to put up or shut up. Events are moving faster than he can control them.
The momentum is all with Trump and the Reps. Mueller must stop it by changing the subject. An indictment of Manafort will dominate the news cycle for days if not weeks.