Unscheduled swaps happen hundreds of times per day for so many reasons. Could be, as mentioned, it needing to end up in a maintenance station. Sometimes it's an ETOPS aircraft that needs to get back to the west coast. Sometimes it's a change of gauge to get a 700 or 800 back on its original a/c line after IROPS. It's a hassle for thrus and crews, but just another piece of the complexity of the network. It's more noticeable at SWA because of thru flights, whereas at other airlines the swap is basically invisible to passengers who have no idea if the tail they are on was the scheduled one.