The protests, as originally started in June, were against a law that would have allowed criminal extraditions to Taiwan, Macao and mainland China. The law was retracted and the large protests have since died down. What is left are a few thousand students who, as advertised in a New York Times op-ed, intentionally seek to provoke the police with "marginal violence":

Such actions are a way to make noise and gain attention. And if they prompt the police to respond with unnecessary force, as happened on June 12, then the public will feel disapproval and disgust for the authorities. The protesters should thoughtfully escalate nonviolence, maybe even resort to mild force, to push the government to the edge. That was the goal of many people who surrounded and barricaded police headquarters for hours on June 21.