The CPC has long meddled in Hong Kong, not least when they disappeared a Chinese billionaire from the penthouse of the Four Seasons [1] who has been 'receiving medical care on the mainland' for 3 years now and 'is very sorry for the trouble he caused'. This is a clear violation of Basic Law and the handover agreement which enshrines a degree of autonomy in Hong Kong and the rule of law.

Then there's the case of the missing booksellers [2]. Another clear violation of Basic Law and the handover agreement which enshrines freedom of speech and of the press in Hong Kong (again, a violation of the rule of law).

Then there's the fact that the high-speed rail station in Kowloon which connects to the mainland is guarded by China Police, who are not allowed to operate in Hong Kong under, you guessed it, Basic Law. [3]

Not to mention the pro-democracy elected officials not being allowed to assume office. [4]

The list goes on, and on, and on. The fact is the PRC agreed to treat Hong Kong a certain way for a certain period of time, and it has failed to live up to its obligations. It has for years now been boiling the frog, so to say, whittling away at the freedoms guaranteed to the Hong Kong people for decades before, and decades to come. With this, they've lurched over the line.

[1] https://www.theguardian.com/world/2017/feb/01/chinese-billio...

[2] https://www.nytimes.com/2018/04/03/magazine/the-case-of-hong...

[3] https://www.theguardian.com/cities/2018/oct/04/high-speed-tr...

[4] https://www.theguardian.com/world/2017/jul/14/hong-kong-pro-...

Also keep in mind that the first strike was probably the annulment of the 1995 LegCo on handover, because it was too open to public influence so the Chinese dissolved it and created a new one in 1998.