In the video, we can see that after the Officer is finished writing out the ticket, he moves to hand the ticket to the man receiving it.
When the man takes the ticket from the officer, he is obviously upset that he just got a ticket, and pulls it out of the officer’s hands quickly.
At this point the Chicago PD officer becomes enraged and assaults the man. He grabs him by the neck and slams him into the wall.
When the man asks why he is being arrested, the officer replies, “you are being stupid.”