Manafort was an FBI informant selected to extract information regarding Russia's influence over the energy market. Many of his actions that are under investigation involve things he did pursuant to this duty.
He was then tapped to be placed into President Trump's campaign in order to give credibility to the "Insurance" policy. The media is and was fully prepared to spin prosecution of these cases as such.
Think FISA. Carter Page and Paul Manafort were both the targets of FISA-1 warrants, which treats the target AS A FOREIGN AGENT. The way they justified this to the FISA courts was to cite their prior undercover work as suspicious.
Which the FISA courts had the security clearance and duty to verify. In short - the judges either didn't care that Page and Manafort were prior FBI operatives, or they didn't do the checks they should have. The result was the grant of the warrants.
These are the actions which Schiff wrote about in his memo - which were heavily redacted to remove the natsec component.
Mueller's probe into Manafort will couple into the investigations into the handlings of the FISA court. This will trigger a massive review of how clandestine programs are handled. There will be a Constitutional Crisis.