This is exactly right, forgive me for not reading the rest of this learned thread. Cavendish is fearless and crazy-aggressive, and pulls wild moves like this pretty frequently. There was just not enough room to Sagan's right, and he tried to lean on Sagan and force him to make room. But Sagan has amazing bike handling skills and immense upper body strength, in addition to the immense leg strength they all have.
He no doubt felt something, didn't even know it was Cavendish behind him, and wouldn't yield, as was his right. The right elbow and left knee were to keep himself upright and keep himself from falling over to his left. Cavendish wasn't strong enough to push Sagan over and fell to his right into the barrier, which was inevitable as there wasn't enough room to pull even with Sagan. I'm a big Cavendish fan, but he got what he deserved that time.