Loads and loads of discussion going on on all the cycling social media at the moment, most people, including me, put it down to a race incident. Both riders are world class and I respect them both but Cavendish always tried to come through forcing a whole where there often isn’t one. As for the elbow… watch the video from the various angles and in slow motion: Cavendish was already going down when Sagan stuck his elbow out (which is a natural reaction that all cyclists do for balance, I speak from experience) Sad day for cycling, a no-win situation, not really, I think it was a knee-jerk reaction and with better analysis of the situation/videos, Sagan would still be on the Tour. (which I think he should be!)
I agree, it looked like a natural balance reaction to me. 
My initial reaction was that it couldn’t be a deliberate act as the one thing that they would all be aware of is that every single micro second of the sprint finish would be covered by film and any incident would be analysed in detail.