It's called a pretext stop. The cop has a legal excuse to stop and investigate this person.
Why did he need to grab her and get her off the bike? What was the "arrest" for that she resisted? "Bike on a sidewalk" is not an offence that would lead to arrest or justify the cop putting his hands on her in the first place.
Too many cops on the street, looking for trouble. That is what happens in a police state. The girl shouldn't have resisted, but the cop should never have put his hands on her in the first place.
At no point does anyone consider that "riding on the sidewalk" does not justify escalating use of force to "enforce the law". No use of force is justified in this case.