Numbers.
When you're outnumbered and the group screaming and cursing at you are getting worked up into a mob, your safety is uncertain.
Rittenhouse was attacked by angry mobs who thought someone said he shot someone. No facts or rational information, just "someone said."
Mobs don't need much of a reason to become life threatening.
He wasn't by himself, there were at least two other cops from what I could see.
None of them were violent or armed with anything that I could tell
None of them were exhibiting aggressive behavior that I could see
If one of them escalates then the officer can escalate.
Plus that was bad tactics and safety. He ran up pushing that lady to a prone position with his gun drawn and in her general direction. If someone is that big a threat that you need to pull your firearm the last thing you do is close distance like that, especially with the firearm drawn. What if she had grabbed the gun? What if the crowd rushed him? His gun should have been in his holster if he were going to go handcuff her.
As for the trailer that seemed needless. Not only did he damage the vehicle, but what happened if he hit the trailer and it spun around or some piece shot off and hit someone nearby? There was nothing in the video that justified ramming the trailer. His overall demeanor looked like he was losing his temper and is not good policing.