Southwest was not in sole possession of your bag for the duration of your trip.
Southwest had access when you checked it in.
TSA had access to it at your departure airport.
Airport employees had access to it as it moved thru your departure airport.
Southwest had access to it as it loaded the plane.
Southwest had access to it as it was taken off the plane at your arrival airport.
Arrival airport employees had access to it as it moved thru the arrival airport.
Southwest had access to it to load it on the carousel.
Who tried to pry your bag open (if that is what really happened)? No one will ever know, but it could have been some person not tied to Southwest at all.
