I was previously told by Southwest that the changes are computer-generated, and the program has some glitches. Even though the changes are supposed to be checked, very often they are not. I once had a connecting itinerary that Southwest changed, but somehow, only the second leg got rebooked, with no way to get me to the connecting flight.I think what happens in those cases is they can only move so many passengers to each selected flight. I think sometimes people are fine with the new flight they were given (or don't even realize they can go back in and play around to move to a different flight) and then sometimes people are like "yeah no this isn't going to work".
BTW, the unsatisfactory itinerary thru BWI is now sold out, but that nonstop is still available.