N723TW flew DL469 JFK-SFO on 1/18 and left the rotation, probably because of a maintenance issue, and is still in SFO (scheduled for this morning SFO-JFK), so DL was down one transcon-757 in the rotation which delayed some of the flights.
Now that this bird rejoins, it should be back to normal.
http://flightaware.com/live/flight/N723TW
Edit: It's no longer scheduled for this morning's flight, so maybe more delays on SFO-JFK flights. Beware.