The aircraft scheduled to operate this flight (N580UW) arrived in SEA 40 minutes late at 6:48 PM. Given the schedule (arrive at 6:08 pm and depart at 7:47 am the next day) my guess is that the crew gets 12 hours of rest in a hotel in SEA and then operates the same aircraft back to DFW. The buffer is probably quite small, so a delay on the inbound aircraft delays the flight back to DFW.

ExpertFlyer says that the delay is due to crew availability as well.

If it gets later, the next AA flight with open seats seems to be AA 1728 at 1:48 pm with Y3. AS 726 at 8:30 is showing Y2 as well.