With SWA you can tell if a flight is starting to fill by the price of the flight. Prices rise as flights fill up. Once you get to the point where there aren't any wanna get away fares, you know it's filling quite a bit. On flights with stops it could be just one leg of the flight that's filling.
OP's situation sounds as though the airline changed planes in the short time between checking in at home and arriving at the airport. Nothing can be done by the passenger to avoid this. Recovery would have to happen at the airport. It's understandable that the airline would do their best to get them seated together again, which they did do. The fact that OP said they were kind enough to get them together suggests (at least to me) that OP was polite rather than demanding which is also important, especially since the person trying to help is not the person who put passengers in this position in the first place.
This really can happen with any airline that has assigned seating.