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Interesting to see the incident in that series of photos. Seems pretty clear after that. Sagan went left, Cav went right, both trying to get Demare's wheel, and they collided. DQ's obviously bollocks, whatever. If you look at Sagan's knees in that last photo (right knee close by the top tube, left knee stuck right out) he's obviously trying to stay on the bike rather than trying to shove Cav.
Look, here's the sequence.
Cav is behind Demare, Sagan tries to force a gap between them and in the process pushes Cav into the barriers. It's 100% Sagan's fault.