They were tipped off
https://www.thegatewaypundit.com/2019/02/exclusive-documents-show-cnn-was-tipped-off-on-roger-stone-arrest-by-29-fbi-armed-agents-leaked-by-fbi/
EXCLUSIVE: DOCUMENTS SHOW CNN WAS TIPPED OFF on Roger Stone Arrest by 29 FBI Armed Agents – Leaked by Deep State!
Roger Stone Attorneys Provide Evidence of FBI Leak Ahead Of Conservative Author and Activist’s Arrest
Just over a week after his televised arrest was carried exclusively by CNN, attorneys for the longtime Trump adviser Roger Stone have sent a letter to the House and Senate Judiciary Committees detailing how some reporters may have received an early draft copy of the indictment.
Documents obtained exclusively by The Gateway Pundit show a copy of the draft indictment without the PACER filing number or official stamps of the court, with metadata on the document identifying it as being authored by “AAW”, who is suspected to be lead Special Counsel prosecutor Andrew Weissmann.