ATL security back-ups into baggage claim (sometimes all the way to the MARTA station) are actually fairly common, since before 1/18. They seem to be tied to the periodic cycles (can't remember their term for it) of Delta flights, along with a couple of big AirTran pushes during the day. I've never hung around long enough to figure out the daily cycle, although it would be great if some FT with way too much time on his/her hands could spend some time and track the lines - sort of like the solunar tables that tell you when to go fishing!