Several videos posted on social media showed a chaotic scene as law enforment officers in helmets and reflective jackets scuffled with shouting protesters, some carrying umbrellas. Many of the protesters appeared to link arms forming a barrier as officers moved in.
You can be arrested just because the cop felt like it. They typically have some time span in which to charge you or they have to let you go.
Cops aren’t the ones charging you, that’s the DA.