No one arranged for Barbara Sampson to be on duty that day, she's the Chief Medical Examiner of the whole Department. She doesn't generally do autopsies, as she's busy with administrative work, so she must have took it upon herself to do the autopsy.
Maybe with a hint from someone in the Cabal, but she wasn't scheduled, as McAfee implies, by someone else. As the Dept head, she makes her own schedule.