In my opinion.
Lets take this from the last kilometre.
There already has been a major crash that has thinned the bunch completely. If you look at the faces of everyone contesting the Sprint you can see Cavendish is comfortable. He has a few glances around to check where everyone is. He isn't completely on the limit and just following wheels.
Having said that...he did try follow the French Champion through a gap that was never there. A golden rule in Sprinting, especially the Tour de France, is never dive for the barriers hard, because this is where crashes can happen. It is so easy for an animated crowd to lean over the barriers and touch the cyclists.
So...unfortunately for Cavendish he has tried to find a gap and its closed on him. It was by no means intentional from Sagan. I mean its not like he looked behind and saw Cav and then closed the door. Sagan was touched from behind by a fast moving Cav and thrown off balance. The kneejerk reaction to regain his balance was lean right and put Elbow out (which never actually connected with Mark).
It sad to see these crashes and its possible Cav was on his way to Victory however you cant blame Sagan for being struck from behind.
Definitely not intentional. I call it an accident. What do you think?