My best guess is the rider was too big. I've seen many people squeeze those restraints down, lock, and then some system in the restraint releases it because it isn't far enough down or something. Should be interesting to find out what exactly happened.
SFOT is losing rides, quickly. With Giant down, that only leaves B:TR, Freeze, Judge, Titan, Runaway Mountain, and La Vibora. I can only imagine how packed the lines will be in the near future since Giant was such a people grabber.