Some guy murdered his pregnant girlfriend and fled to Taiwan. Because of Two countries-two system arrangement between China and HK, he couldn't be extradited, so HK government passed a new bill what would close this loophole. BTW this new law could also be used to prosecute HK oligarchs under China's new anti-corruption efforts.

This was used as a catalyst to complain about lack of "democracy" because TL;DR the average economic situation in HK is getting worse and worse. Inequality, rent, living expenses, all of it is getting worse. HK is also already extremely dependent on China for certain things like utilities.

This deteriorating situation is the perfect storm for frustrated HK nationalists(fueled by capitalist interests for further autonomy) to conflate nationalism with neo-colonialism, they literally were begging for the UK/Trump to "liberate" HK, yes they are really that stupid. You can find dozens of pictures of protesters with UK/US flags. Regardless, there is some statistics that showed the majority don't support these protests and are not separatists. Worst yet most of the arrested protesters were teenagers/students. A group remarkably easy to manipulate without proper education. Do you think HK youth are aware or give a shit about what the UK did in India or what the past 20 years of American "liberation" in the middle east?

BTW look at how this protest was portrayed by the media compared to the Catalan separatist movement. Did you see #FreeCatalonia trending in the MSM or liberals? Yeah me neither, obviously Spain is part of the imperial core, a "democracy" blah blah it is inconceivable that people simply don't want to be part of it anymore right?

Eventually these protests fizzled out, the CCP did not send the PLA or do anything that would justify "retaliation", it mean the international community, eventually doesn't have much to support, since there have been very few deaths etc. Some leaders fled, others were arrested, and in general the vast majority of the population in HK supports China, because these protests, valid or not, made the economic situation even worse with the loss of business and tourism during the period. There have been numerous polls, before and after these protests and the majority of HKers are not separatists and do not support these protests.

Of course this sort of struggle is perfect for color revolutions, so the anti-China interests saw a huge opportunity to use this(don't forget this was still in the middle of Trump's "trade war") to weaken China. Clueless Westerners fell straight into the narrative of "bad authoritarian CCP bad" or whatever, I am pretty sure 95% of people in the west, when questioned over their support for the HK protests, have no idea of its origins, ties to neocolonial/separatism/terrotism etc. Indeed western media did an excellent job, unfortunately, controlling the narrative, as the true origin or necessity of the extradition bill is never explained.

In any case the extradition bill was not passed anyway.

Anyway I really recommend reading the r/sino FAQ on HK protests, it is very comprehensive.

Now going a bit beyond the scope, I think it may be fair to question "what if I'm leftist and I want HKs social/economic situation to improve/stop deteriorating?"

The answer clearly isn't separatism, but rather closer integration with China. Forget China's recent reduction in poverty and booming economy, the realistic point of view is that HK's days are numbered as China is already seeking to replace HK as an important center of finance. Like it or not, it is either further integration or becoming a puppet state/colony of the imperial powers.

In short, even if independence was a thing, realistically, it would be a very very bad thing. This isn't a Brexit situation, China isn't a neoliberal institution forcing austerity down peoples throat and bankrupting nations(e.g Greece). HK's destiny, if independent would be to be a pawn of imperialist western powers just to be used against China and nothing more.

And while right wingers are deluded enough to think that is attractive(who wouldn't dream of an UK passport?) the reality is obvious, HK would become a puppet state, the economic situation would degrade even faster, and anyone hoping to emigrate will find it extremely hard given the recent fascist trends inside the UK. Being Asian and thinking you will be welcomed back in Britain amidst Brexit chaos is fucking delusional and self destructive.