A family friend who is a travel agent was stranded in Denver for about twenty four hours by United Airlines and became our house guest for a while. While it is always nice to have an unexpected visit from a friend, the reason for the delay was unusual. There wasn't a mechanical problem, and there wasn't a weather problem. The United crew simply decided that it didn't want to fly the flight that she was scheduled to take.
Apparently, the crew has this authority, and as long as it isn't used frequently, I can imagine situations where it might be a good one -- the "something seriously doesn't feel right, but I'm not quite sure that I can put my finger on it" kind of situation. Then again, one can also imagine room for abuse of the privilege, particularly by employees who are understandably unhappy with an impending bankruptcy settlement that gives executives fat pay boosts, while stiffing rank and file employee pay and retiree benefits in stunning amounts.
I doubt that the mystery behind what stopped this particular flight will ever be known.