1. There was no problem as long as people stayed on the sidewalk.
2. A small number of protestors left the sidewalk and took to the street, chanting that they were going to take the bridge.
3. Police told them to get out of the street or they would be arrested.
4. At some point, the police were walking in the street, and the protestors in the street were following them.
5. People on the sidewalk farther back or not yet on the bridge saw that, and thought the the police were given the protestors an escort in the street and that it was OK therefore to walk in the street.
These protestors really need to get better organized. When they decided to head across the bridge, they should have made it clear that the plan was to stay on the sidewalk. The leaders knew this, but apparently it was not communicated to the masses.