Some more details...
After it happened, someone...I believe a former employee of a Six Flags park with a similar coaster...explained to me exactly what happened, and it makes perfect sense, particularly now that I've seen a Schwarzkopf shuttle loop.
On the back of the train, there is a counterweighted arm which is pulled down with an electrical solenoid. With the arm down, the pusher is engaged with the back of the train, and holds the arm in place. The counterweight tends to cause the arm to swing upward, away from the pusher. The idea is that the pusher shoves the train down the track, then when the train overruns the pusher, the counterweight lifts the arm. That way, when the train comes back through the station, the arm won't hit the pusher. Gravity holds the arm up, out of the way. Even when the train is going through the loop, the forces on the train pull down on the counterweight, holding the arm up and clear of the track.
Through most of the ride, the arm will easily clear the track. But there are two spots, at the entry to and exit from the loop, where the arm can touch the track ties if the arm is down. Apparently, the train launched with insufficient speed. As the train went into the loop, gravity overrode the centrifugal force, so gravity lifted the counterweight, lowering the launch arm. The train didn't have enough energy to make it through the loop, so it started to roll back. The push arm, being pulled down by gravity acting on the upside-down counterweight assembly, drops down, and catches on a track tie, jamming in place. This prevents the train from rolling backward, while the mass in the back of the train hanging down in the loop prevents it from rolling forward. This also explains why, as the train was unloaded, it would roll forward: Take the riders out of the back of the train, its center of mass would move forward, and eventually the mass of the front of the train should bring it down out of the loop.
I understand that when this happened to an unloaded train many years prior to the Walibi incident, the park's solution was to weld a piece of flat bar stock all the way around the inside perimeter of the loop so that the pusher plate could not catch on anything.
As for hanging upside down from a lap bar, try an experiment. Get a chair or stool, and balance yourself face-down on the seat by lying on your chest. Notice that you can't do it. Next, try balancing yourself face-down on that same seat by lying on your pelvis. Notice that it isn't terribly difficult. The difference is that you are supporting your weight at or near your own center of mass. If you do the same in a roller coaster car, you can dangle riders far more comfortably if you support them at their center of mass instead of well above, where it can be downright painful.
--Dave Althoff, Jr.
Who has dangled from lap restraints a time or two, including once in a '95 Taurus...