Flew IAD - ATL on Delta Friday. Getting through IAD was easy, no lines at all. Flew standby on an earlier flight, with a checked bag. Agent at check-in used a green standby luggage tag. Cleared standby list and bag arrived on new flight. We did pull back from the gate 1/2 hour late. Pilot made announcement that it was due to bag matching. ATL security lines were back up to baggage claim at about 4:30pm. Also, overheard two Hartsfield employees conversing about check in lines being backed up into the baggage claim earlier in the day.