Manafort had his bail revoked for contacting potential witnesses and asking them to lie to investigators.
From an NBC News article:
But prosecutors made a new bid to toss him behind bars after uncovering evidence that allegedly shows that he and a Russian associate, Konstantin Kilimnik, used encrypted messages to reach out to two witnesses. Mueller's team said Manafort wanted the witnesses to back up his claim that prominent Europeans he hired didn't lobby for Ukraine in the U.S., which would be illegal. One of the witnesses felt it was an attempt to "suborn perjury," prosecutors said.