It seems Trump has had "Enough" of Sessions, now that Trump's more sure that the replacement's nomination will be confirmed by a Senate more dominated by Republicans who owe their seats to Trump's popularity in their districts. (In January, Republican critics of Trump will be fewer and farther between in the Senate.)
Also, Rosenstein is no longer overseeing the Russia probe. That will be overseen by the new Acting Attorney General, Matthew Whitaker, who wrote
an op-ed piece titled "Mueller's investigation of Trump is going too far".
Ooooookay.