Well, when I look at the seats and the restraints and put it next to the picture of the woman, I'm pretty sure, the station workers had to press pretty hard to get the restraint to "clack" just once (Actually, in an interview with a witness, they said it in deed locked just once and that the woman didn't feel safe about that). And that's, what shouldn't have been ... When it's that hard to lock the restraint, that person shouldn't be on the coaster.
I was on a Top Spin once and I kinda like pressed the restraint down a too much and during the ride it released itself a little bit. Shock of my life, but nothing happened. I think, that's what happened to the woman due to her size and the negative G-forces. And because it was only locked once ... well, that happened.