I think a few things happened here, starting with Cavendish running into Sagan and lunging at him to make room where there was never going to be much room. Sagan could have obligingly moved over to let him past, but why should he? It’s a race. Then Sagan’s elbow gets hooked over Cavendish’s bars. By this point, Cavendish is falling into the barriers, dragging Sagan’s elbow out (making it look like Sagan is elbowing him).
Meanwhile, Démare swerves in front of Bouhanni, impeding the latter. Not much comment about that, I notice.