Actually, I'm surprised it doesn't happen more often. Some of those multi-story car parks have only a flimsy pedestrian barrier that you wouldn't want to even lean on, let alone push a car against - you wouldn't even feel it.
What's missing is a concrete stop strip. I have no idea how this is legal, but I used to park right on the edge of a rooftop, exactly as I described. It gave me the heebie jeebies driving up to a precipice like that, and particularly when leaving, making extra sure I had selected reverse gear ... But then I have an imagination and tend to be aware of hazards around me.
So imagine yourself twisted around in your seat looking over your shoulder, you push the throttle pedal with your foot, and from your perspective the car lurches backwards. It's confusing and a shock. 99 times out of a 100, you'd manage to find the brake pedal and just be embarrassed. But if your footwear was feminine, or maybe you're a little shorter than ideal, it would be easy to take a second or so longer to bring the car under control.
If you hit the brakes just before the front wheels go over the edge (a distance of 4 or 5 feet in some of these carparks), the weight of the car will be shifted to the front and the rear wheels may lift, negating your braking. You'd just slide over the side, which may well be what happened here.