It is because the police are not arresting these Democrat terrorists who have crossed the line to harming people and property. I think it could get violent as the police are not enforcing the law and as these Democrat protesters see more of that then people on the other side will be defending themselves - it won't end pretty.
That is likely not a coincidence. Mayor Liccardo let the cat out of the bag when he blamed Trump. These Democrat mayors are on the side of the mob. Their seemingly ineffectual police are lead by Democrat political appointees who restrain them from action.
It is a sly wink and nod from the Democrat political establishment that low-level violence will be tolerated as long as it is directed against the right people. In this case, Trump supporters.