I think its a very naive take to think that Tony wasn't angry and having one of his temper tantrums when he hit the ill fated acceleration. Anyone who has even a casual knowledge of Stewart knows about his temper. 

 

As suggested earlier, he likely was trying to spray the other driver with dirt and carelessly did not take into account that the back end would swerve upon gunning it. 

 

It was horribly negligent and he deserves whatever punishment he gets. 

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