Someone called 911 and reported that these demonstrators were throwing leaflets off the overpass and that a flag fell and hit a car. The police are required to respond, if someone calls 911 and they do not already know about the event. O-bots across the country use this to try to disrupt the demos. Cops are often irritated about being called out and may blame the protesters.
If you let the police know about the event in advance, they may not have to respond at all, depending on what the call is about. You should also let them know that such false 911 calls are often being used in an attempt to disrupt these protests to turn their irritation back on the caller. It is illegal to make false 911 calls.
It is best to not do anything that even appears to be unsafe or presents a risk of something falling into traffic, so that the police know that there was no basis at all for a 911 call.