Bill Palmer might be on to something here. He speculates:
Special Counsel Robert Mueller has likely had Rudy Giuliani nailed since last year, when he cut a plea deal with Giuliani’s former client, Turkish criminal Reza Zarrab. Since that time the question has been whether Giuliani has secretly cut a plea deal against Donald Trump. Believe it or not, the FBI’s sudden raid against Trump’s attorney Michael Cohen provides strong evidence that Giuliani has indeed flipped on Trump.
In January, Donald Trump nominated Geoffrey Berman as the new Attorney General for the Southern District of New York, which has jurisdiction over New York City. Berman had been law partners with Rudy Giuliani. This is presumably why Trump picked Berman, expecting that Berman would protect everyone involved in the Trump-Russia scandal. But it was Berman who signed off on the FBI raid against Michael Cohen.