This was a political attack by Mueller and his team, aimed at inflicting the maximum damage on Trump's presidency. Its goals were: 1) to continue the "investigation" as long as possible to damage the President's image and to undercut any attempts to charge the Obama / Hillary / Deep State spies, leakers, and perjurers, and 2) to bait Trump into doing something they could plausibly call obstruction.
With Rosenstein unwilling to put any boundaries on Mueller's activities, the only way to end his quasi-legal reign of terror was by replacing the Attorney General.
… replacing the Attorney General.
And they are masters at that. E.g. Trump Tower meeting with a Fusion meeting with the same people the day before, the day after, and (I read here recently) the day of. E.g. Halper and Mifsud and Downer, etc all offering collusion bait.