What happened to the Guttery family at the dock? How did this incident point out the change in attitude of some of the coolies caused by the revolutionary movement?

The Guttery family are threatened by the coolies—unskilled manual laborers—who work at the dock. They're engaged in a revolutionary uprising and see this American family as members of the hated bourgeoisie whose economic and political power they're dedicated to destroy.
Previously, the coolies had been meek and humble workers, putting their heads down and getting on with their work day after day. They were friendly and deferential, not the kind of people you'd normally expect to act in such an aggressive, threatening manner. But under the influence of Communist revolutionaries, their whole attitude to their alleged social superiors has changed. They're tired of being on the bottom rung of society, sick to death of being treated by Westerners like second-class citizens in their own country. And so they rebel. Unfortunately for the Gutterys and many others like them, they find themselves caught up in the middle of this monumental social and political upheaval.