I believe irregular operations basically means the entire system is messed up. Could be weather disrupted ATL which would throw all flight schedules out of whack for the rest of the day - afternoon thunderstorms are great for this in the summer in ATL. Could be a computer failure like NW/Pinnacle had last week where everything was running hours behind. Aircraft are out of position, so they are trying to get things back to normal.

I had the same in ATL last fall. Was late getting out of HSV, trying to get to DTW. Every DL flight to anywhere in the eastern half of the country was late. Checked with gate agent to see if they could route me through CVG instead, but that had fewer backup options. Got to ATL about two hours late. There were several DTW flights on the ground, waiting for gates, switching gates, .... A complete mess. I now avoid ATL, especially in the summer.