Pilot Ray Stark has offered his personal explanation at the official Southwest Airlines Blog. He says that pilots have a key decision to make when pulling away from the gate.
It's up to the pilot to listen to reports from air traffic control and judge whether or not they should let passengers board the plane. His personal rule is that if it sounds like air-traffic control won't let the plane take off for another hour, he won't let passengers get on board. But sometimes air traffic control provides a faulty forecast.
This pilot's explanation is helpful, but it's worth noting that even he thinks that an hour long delay on the tarmac is, well, highly undesirable. To put his post in context a couple of weeks ago, a Southwest spokesperson told a group of reporters that the airline opposes any federal rule mandating that planes must take passengers back to the gate if an airplane has sat on the tarmac, even if it's for specifically rare and extreme delays of three hours or more.