I thought perhaps Button twitched to the left as Vettel tried to get alongside and that was what caused Vettel's overreaction and subsequent crash.
Either that or a bump (according to Vettel) or perhaps simple physics - once he came out of Button's tow front wings started to work again and gave him too much left steer, which caused unfavorable reaction at the rear etc. Most probably all of the above packed togethe.r
GREAT race though, it'd be even better with the above two.
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I wonder if jumping jack flash dirt racer guy did it on purpose?