It's both of their faults.
The guy on the track for getting out of his car and walking towards a deadly weapon (car), and Tony's for burping the engine and causing the fishtail.
I personally think Tony meant to get close to the guy and scare him, but not hit him, and the guy definitely walked towards the car more than he should. I've seen this countless times at my local dirt tracks and have done it a few times myself (both sides).
This is the correct answer.
It's not black or white. It is both of their faults.