Sullivan apparently said Flynn was not guilty and they gave him multiple opportunities to challenge the FBI interview and withdraw his guilty plea.
Catherine Herridge was in the hearing and just said he asked him to withdraw his guilty plea twice and Flynn refused.
Basically Flynn is scared to death (spooked) of Mueller either going after his son or most likely charge him with the whole Turkey mess so he WANTS to plead guilty and get it over with. Sullivan and the whole D.C. knows Flynn is not guilty of lying to the agents but Mueller has him dead to right