Having worked inside a health insurer, what that usually means is the Dr (or rather, his overworked assistant) screwed up the coding on the forms submitted, and it hit some kind of auto reject. It's mostly automated, or was at the place I was at (~90% or more).
Given there are a ridiculous number of codes today, and IDC-10 is going to drop a nuclear holocaust level of them, it's going to be interesting to watch from the sidelines. Doctors aren't going to have a hope in hell of getting them right, and insurance companies aren't ready to deal with the 150,000 new codes. Idiotic bureaucracy and healthcare, yay!