We all know it's a risk to race your car. And if you cause an accident at an historic event you don't get invited back for 13 months! But for this I'm sure they took someone out back and beat the crap out of him too!
There is lots of commentary at the start of the post linked above. Looks to be confusion on the pack of racers with regards to a green flag, speeds, and hills. Chain reaction with those at the end of the pack approaching at high speed on those at the front that were at a lot slower speed.