City,
SWA1380 had filed for FL380, and was climbing through FL325 when the immediate descent began. At this point, the airplane was approaching the Susquehanna river in southern Pennsylvania. The closest suitable airport with airliner services and emergency services was MDT, but they needed a distance for the descent.
PHL was a good choice as they had to go that far anyway during the descent, and SWA has a large base at PHL. BWI would have been another option, slightly further away.
You can view the
lateral flight track and the
altitude track.