The Chinese central government is not happy to let Hong Kong run itself. That's why Article 23 was pushed numerous times, as well the National Anthem Bill and the extradition law. They put their customs officials on Hong Kong territory, to the dismay of many locals. They also conduct extra-judicial behavior such as kidnapping HK citizens.
In 1997, HKers were afraid their freedoms would erode under Chinese rule. We were left alone for a while, but the spectre was always there. Now, the Central Government has made it more of a priority.
That said, there are many HKers who consider HK part of China and many others who don't.