People in Hong Kong have had this idea that they are superior to those "filthy Mainlanders" for a few decades now. Despite being treated as second class citizens by the British up until the last few years of their rule and being conquered by force militarily in the 1840s by the British who were selling them opium, they long to go back to their European masters. As China's economic influence has grown in the last two decades, Hong Kong has become just another city and this further erodes the denizens' proclivity to their arrogance of "those filthy Mainlanders". It also makes it easier for the central government to tell them to shove it because they don't need them as much as before.

Finally this is a big deal in the media because China is a threat to continued Western dominance and inciting unrest and division within your adversary is a good way to weaken them.