I suspect Manafort has some legal problems having nothing to do with Russian interference by the campaign that Mueller wants to use to put pressure on him to spill his guts about the Russians.
I truly believe there is nothing to spill about Russia. Instead it’s going to be about setting up a perjury trap, remembering that this FBI seemingly employed by anti Trump political forces.
This AOL article explains a number of reasons for using the no-knock warrant. One mentioned is that when this kind of warrant is executed one cannot use Fifth Amendment protections. Also many comments, some informative.