Zinn's article mostly makes sense.

1. He did shift right before the incident. http://www.youtube.com/watch?v=kaLq1yo0hIs Wait for about the 25-second mark when they blow him up in the white circle. Watch his right arm. Then watch the RD.

2. It doesn't look like he moves his left arm, so I don't think it was a big ring/small ring issue.

3. He didn't hit a bump. You can see from the video that the hop in the rear wheel is a result of the chain getting stuck. The front wheel doesn't appear to hit a bump.