To illustrate what has happened:

Vettel makes his over exited over taking move.

Vettel's left front wing shoot into full airflow, producing unbalanced huge down force on his left front tyre, the car head rapidly towards left. Vettel initiated suddern move towards left, car was unforgiving and enhance Vettel's imput.

As the front end of the Vettel's car come completely in the air flow Vettel tries to counter by steering towards right agressively. Front wing rebound and right side flex, ending up on higher down force on right front tyre. As the car rotate clockwise, rear wing loses much down force which accelerated the rotation.

Vettel steer left to counter rapid rotation. Front wing bounce back once again. Vettel over shoot gas as he tries to put his car on track, using Button's car as moving hand rail in order to place his car back in position.

Vettel get his car back on track successfully reducing the speed of the car. Button got huge damage on his left radiator. By now Vettel has managed to take Button out even if it were pure consequence of his effort to keep his car on track. It was just sequences of reflection.

Button ends his race, Vettel change his nose and continue.

Vettel excuses himself later on by phone to Button.

Now whats your excuse for the fact that no one else has crashed while following an F-duct equiped car?

Are you saying that this was caused because the car that crashed had an f-duct, or by following a car with an f-duct?

Not sure if he was picking up on this post of mine but anyway...

I believe the car oversteered like that because there was too much downforce on the front relative to the rear. The reason there was not enough downforce in the rear is that the car has an f-duct -- a device specifically designed to reduce downforce in the rear during straight-aways in order to gain an advantage in speed. The reason the Red Bull has an F-duct is that McL has an F-duct and so, in order to be competitive, all the other cars have to have F-ducts. It was a loophole in the rules which McL was able to exploit. But there's a consequence and that is that once the other F1 cars have adopted the trick, all the cars this season spend large amounts of time on every track with a serious imbalance (and general shortage in rear) of downforce.

This might not seem terribly unsafe on surface, but I believe it does reduce safety in F1. There are two reasons for this. One is that it requires the driver to devote attention and even a hand on the steering wheel to the act and timing of act of moving parts of their body to activate the f-duct. This is a relatively pointless opening for error (if the advantage of the tech is canceled out by everyone having it, then all that's left is everyone has the dangers of it -- inconsistent downforce). Additionally, I think it's very dangerous to be having the drivers taking hands on and off of the wheel right at the time of passing -- it is a source of instability and reduced control and ability to make responses in steering wheel. The other is that it makes it so if the driver makes a maneuver with the car, either for emergency or for passing or whatever, unless he has already successfully gotten the rear wing to reactivate completely, the car's handling will be severely compromised and unpredictable.

We see in the video that Vettel clearly was having to use the F-duct in order to not be too slow to pass Button (who was certainly using F-duct). But this required him having his hand off the steering wheel until the very last second. The combination of rather abruptly switching hand from F-duct to steering wheel combined with the F-duct having removed downforce in rear but being expected to be switching to high downforce although in, by nature, impossible to completely predict way (especially when the car is in dirty air of other car), I believe is what caused the extreme oversteer and snapping of car into crazy crash.

sums it up perfectly