This is a gif made up of individual frames between seconds 20 and 21 from the video above.
The gif pauses on the frame where you can best see Cav's hood hooked on Sagan's right wrist/forearm. From there you can watch his arm get pulled away from him. Notice his core is directly over his bike until the later part of the gif. That's because Cav's bike is pulling Sagan over towards to his right. Once Cav's bike releases from Sagan's arm, you'll see Sagan's arm drop down a bit then stay out as he regains his balance. If you've ever messed around with your bros while biking and you want to deflect their path, you put your weight behind your shoulder or arm. Sagan's weight was pulled towards Cav as he went down, due to his bars hooking Sagan's arm.
Click on the video above at second 19 and go through it click by click on the slider. Cav rode up from behind and the gap began to close. Cav appears to lean into Sagan while giving him audible warning of his position. At the same time his bars ride under Sagan's arm and gets hooked. Sagan's elbow comes up in order to get his arm over the hoods. Once it does, Cav's wheel turns right and he plows into the crowd barrier.
To make Sagan look even worse is his extraordinary biking handling skills. If it had been another rider and they went down, we'd likely be talking about how reckless Cav was with that move. Instead the UCI took action without proper investigation or even interviewing either rider.
This is the frame where you can best see Cav's bar hook Sagan's arm. Go back to the video and look at the shadows on the ground to get the sun's location. Cav's hood is lighted by the sun which means its at least even with if not further forward than Sagan's arm, otherwise it would be in shadow.
