I think you may have answered your own question, in the sense that China took this opportunity because the world is so preoccupied with the pandemic. However, there is some history here. In 2003, they tried to introduce something that was actually, if anything, maybe even a little milder than what we're seeing now. Protests in the streets discouraged the Hong Kong administration from proceeding with that. Many years have passed since then.
My own reading is that last year Beijing lost faith in the Lam administration in Hong Kong and decided they would simply interfere in the system. One thing to remember is that, as I talked about, China would attack Taiwan if they felt their interests were imperiled. As valuable as Hong Kong is to them, they will quite happily see Hong Kong's future blighted to make sure they maintain control over Hong Kong. This is precisely what we're seeing.
What's serious here is that we're seeing the Chinese legal system penetrate the barrier that had kept Hong Kong separate—the notion that people who are accused of national security violations, which are very broad and ambiguous, could in fact be renditioned to China for trial.
So Canada was wise, I think, to cancel extradition. It's really, as I say, the death knell for one country, two systems.