Somebody on the transition team. Sessions? Rosenstein had a respected background and looked good on paper. My guess is Rosenstein wasn't the guy to handle a s**t storm. He justifiably fires Comey and then panics with the selection of Mueller when there are calls for a SC. My guess is Rosenstein was working from the premise that there might be something to Russian collusion, so he allowed the investigation to proceed. He wasn't on the job very long when he appointed Mueller. Many of the bad actors are still in place at DOJ and FBI and he isn't going to get any info out of them that Russian collusion story is bogus. I am guessing most of the recent revelations have come from the OIG investigation at DOJ and unannounced ones at NSA and CIA.
Still don't know whether Sessions' recusal was idiocy, stratagem, or necessary for political appearances / to preserve prosecutorial independence.
Also, Mueller and Comey have been friends for a long time; several of the top people,including Mueller and Comey were deeply involved in Uranium One; and in addition to one Federal Judge who had to recuse themselves, there is another Fed Judge, a last-minute Obama appointee who served on the Harvard Law Review with Obama (and who blocked one of Trumps EOs), who magically got assigned one of the earlier dirty cases, and one of the related new cases -- I think his last name is Chang.