calgarygary
Southwest has that policy in there by-laws. But they never do it. I have flown them 2x a month for the last 18 months and they have NEVER changed a flight time or cancel a flight because of not enough passengers. There schedules never change. Only weather conditions would alter a Southwest flight schedule.
In fact on several occasions I have been on a Southwest flight(Pittsburgh to MCO) and have had less than 20 passengers on a plane that holds 128. Most of the other legacy airlines would cancel a flight like that, and move the passengers on another flight.
Most likely, Southwest needed that plane at the new destination for the next round of flights. My dh flies out of BWI each week and on more than one occasion has seen a Southwest problem. I know one time there was a flight to Orlando that was completely cancelled at the last minute leaving a terminal full of people not arriving until the next day. It happens, usually due to weather, and no airline is immune.