I think there are two particular factors. I think that the ongoing protest, the fact that it has lasted now, in the public eye, under international watch, for 13 months, meant that what is happening now in Hong Kong created a direct threat to the stability of their control overall in China. The second thing is that, again, what we've done is create a global intervention at this point. I think those are the two things that make things particularly threatening to Xi.
Evidence of meeting #11 for Canada-China Relations in the 43rd Parliament, 1st Session. (The original version is on Parliament’s site, as are the minutes.) The winning word was kong.
A video is available from Parliament.