IMO, Petacchi didn't keep his line and went to the left; which is OK because he already passed Van Avermaet who couldn't keep up the pace. McEwen however, in his effort to keep Petacchi's wheel, did not take into account he could not go to the left as well because Van Avermaet was still there. So he touched Greg who was blocked by the barriers on the outer side. It was a blind sprint by Robbie.Alpe d'Huez said:Here's a video of the last kilometer. When you get to the site, you'll see the "laatste kilometer" link to the right of the window. Click it.
It looks to me like Petacchi was borderline at fault. You see it in the overhead shot. But it's a tough call, Van Avermaet may have caused it mostly on his own as he was the first guy to go down.
A bit of a warning: This is an awful accident. Robbie and Tom look like they were hurt pretty bad, a lot of asphalt burns and skin scraped away.