According to the initial news report the source was one or more military officers:

Commander disciplined for nuclear mistake

By Michael Hoffman - Staff writer
Posted : Friday Sep 7, 2007 19:02:15 EDT

The Air Force continued handing out disciplinary actions in response to the six nuclear warheads mistakenly flown on a B-52 Stratofortress bomber from Minot Air Force Base, N.D., to Barksdale Air Force Base, La., on Aug. 30. The squadron commander in charge of Minot’s munitions crews was relieved of all duties pending the investigation.

It was originally reported that five nuclear warheads were transported, but officers who tipped Military Times to the incident who have asked to remain anonymous since they are not authorized to discuss the incident, have since updated that number to six.


http://www.militarytimes.com/news/2007/09/airforce_nuclear_warhead_070905/

Other comments on the web suggest that they reported it because of thier concern over: 1) unauthorized movement of nuclear weapons; 2) concern with personnel outside the regular chain of command seeking to bypass normal checks on weapon movements for purposes unknown.