This happened for 2 reasons, ignorance of the situation and lazyness:
1.Lazyness: they are climbing on the wrong side of the rope (the one not clipped, because they where too lazy to pass all the rope trough the anchor and belay from the orher side, so that the climber could climb on the clipped side of the rope), and that wouldn't be a problem except when:
2. Ignorance: they are climbing on a overhanging wall (direct from the anchor) so the diagonal distance from the anchor to the climber beats the vertical one to the floor (that is pure lack of awarenes and when combined with 1, makes this nasty shit happen). I'm glad she managed to climb that high or she would have hit the floor without any pendulum.