Flaps on big jets have mechanical links that freeze their position if a significant asymmetry is sensed during movement (stopping before the asymmetry is enough to notice). But then the flaps are frozen in position which explains their return to the same airport. If the flaps are frozen in a near- retracted position then the approach speeds will be much higher than normal (even after dumping fuel), leading to other problems such as landing distances too long for available runways, high angle-of-attack approaches at high air speeds, hot brakes, and possibly even blown tires.
The nature and seriousness of the problem was likely unknown to the passengers.
Maybe the passengers who stayed on the airplane had been drinking and were afraid their alcohol consumption would be noticed by the Saudi authorities if the got off the airplane!
I know, the flaps shouldn’t go asymetric, but sometimes they just do. http://avherald.com/h?article=45a126c3
But if the solution is to land the plane, why not land it at its destination after burning off the fuel for a purpose rather than dumping it?