I've watched this
video several times now. It's really hard to tell if Contador could see Schleck with Vinokourov in the way. I'm willing to give C the benefit of the doubt. (Even though I'm rooting for Schleck and I hope he stomps all over C tomorrow.)
What about a theoretical scenario of Schleck's shoe coming unclipped and he was having trouble getting clipped back in? Similar issue, and I wouldn't expect anyone else to stop and wait. From the video, it's difficult to say for certain if C could have been able to ascertain there was a chain problem. At any rate, he was finally up to speed and possibly focusing all attention on his efforts.