Could've happened too fast for the instructor to realise. Or perhaps the instructor did hit the dual controls, and that's what tipped the car? It sounds from the reports like she accelerated hard while turning right at a T junction - I guess if the instructor then stood on the brakes the wheels locking could've tipped the car, or he might not have had time to get to the brake pedal before the car hit whatever turned it over (although if you look on Google Maps it's hard to see what she could've hit that would flip a car doing the kind of speed you'd go round a corner at...)