Once Trump had picked Jeff Sessions, he was up a tree when it was revealed that Sessions would not pursue the “Russia probe” shutdown. Once the mid-term elections were cleared, however, and there was a good chance of seating a nominee who could pass muster with a Republican-majority Senate, as the optics of turning Sessions out of office were much better.
In the sweep, Rod Rosenstein (the real target, so far as Trump was concerned) could be cut loose as well, taking away that last level of protection under which Special Counsel Robert Mueller was operating.
Mueller may take the direction of the investigation either in the direction of where the REAL Russian collusion was going on, or wrap up and get out of town.