Previously I was thinking, that Sagan has blocked Cavensdish, but after I have rewatched crash videos, I have changed my mind.
Here are pictures showing movement of riders until contact between Sagan and Cavendish:
https://photos.app.goo.gl/ueB6AgZ8cHvklkLS2
1 - everything OK, every rider is in his line
2 - the moment just before Demare starts his move to the left side, Sagan seems to be closely watching Demare.
3 - Many people still think, that Cavendish was on Demare's wheel and Sagan just took it. On pictures 3,4 you can see what trees hide, when watching from above. The fact that Demare was moving to the left side already in this moment changes a lot, when judging Sagan's movement. Cavendish lost Demare's wheel, because Demare left his line! Sagan reacted immediately, as he moved at the same time as Demare, to get into his slipstream. As a result Demare's and Sagan's lines united in one.
4 - Here are some bad luck factors for Cavendish: drain hole and spectators. Probably he would survive without crash, if they weren't there (despite he didn't brake, when he couldn't follow Demare's move to the left and additionally his space got even tighter as Sagan jumped into Demare's wheel).
5 - If not already earlier, here is clearly visible contact between Sagan and Cavendish. Further Sagan's action was strongly influenced by this contact as he was regaining balance and control over his bike...