The issue was apparently drift in the actuators that positioned the second stage engine.
Where did you get this information from?
"During the terminal count engineers observed drift on one of the two thrust vector actuators on the second stage that would likely have caused an automatic abort. Engineers called a hold in order to take a closer look. SpaceX is scrubbed for today and we are now targeting launch on Jan. 9th at 5:09am ET."
Both the nasaspaceflight.com forum and the Nasa TV webcast.