Hong Kong anti-government protests are continuously developing without a stop sign for the past two months. If you want to catch up on the story, you can watch this video made by Vox.
Although the statement about China Mainland is conventionally stereotyped, the other parts can help understand the previous story.
At first, the concern is Hong Kong people fight for their right under one country, two systems framework. They objected the extradition bill, which would also allow extradition to China Mainland. Since some people have low confidence about China judicial system, they do not believe China mainland have a fair trial. In spite of this, Hong Kong government has the obligation to explain the detail of the extradition bill and free people from anxiety. The activities were still legitimate.
However, on July 21st the meaning is changed when protesters defaced Chinese emblem at the central liaison office, the representative of the China government in the territory. 
On Aug 6th, Beijing warned that Hong Kong’s protests had gone beyond free assembly.
Hong Kong is certainly influenced by China Mainland passively. Since the growing economic gap between China Mainland and Hong Kong is becoming bigger, Hong Kong loses importance to the China Mainland. In 1997, Hong Kong’s economy was more than 18% of China’s economy. After two decades, Hong Kong’s share of China’s economy is down to about 3%. 
Hong Kong is no longer irreplaceable to China. It has much less weight on the entire China economy. Shenzhen had achieved explosive development just next to Hong Kong. In 2018, Shenzhen GDP surpassed that of Hong Kong.  The city becomes fertile ground for high-tech companies such as Huawei and DJI.
But Hong Kong is the gateway to the West since its investment and trading environment. There are 253 Chinese companies from bank to tech firms raised capital in Hong Kong’s stock market.  In 2016, Hong Kong’s share of total FDI in China mainland was about $81 billion.  From 2014 to 2016, Hong Kong established a total of 38,068 FDI firms and about $241 billion in the China mainland. 
Beijing has still expressed their belief that the Hong Kong government and the police have the ability to handle the protest. But massive armed police were deployed in Shenzhen for any further action.
 Hong Kong residents deface Chinese emblem in latest protest
 The World Bank data
 Shenzhen surpasses HK in GDP
 China dimension: list of H Share Companies
 Report on Foreign Investment in China 2017