Realization they're not in charge is likely the main driver I would imagine.

Well, you just need to read between the lines there.

uncertainty over China's economic trajectory

Read: uncertainty over ability to continue exploiting labors using your accumulated surplus

geopolitical tensions...

Read: closing window of opportunities to freely move your accumulated surplus elsewhere

This is just like those immigrated nobles running from liberal revolutions in the past.

Being wealthy in the US/UK/Canada is so much better than being rich in China because the marginal advantage is substantially larger

In China? You still take the train. Transit is probably still faster than driving. Your food expenses don't scale as fast because cheap food isn't made of garbage. You already likely own a home. You probably come from a tier 1/2 city, so traveling domestically is cheap. Your kid still has to grind for gaokao.

In the US? You fly business. You drive around in your S Class. You can bribe infinite extracurriculars for your kid to get into a top university. You basically never have to interact with anyone in the lower 90% of incomes, and you don't need to be THAT wealthy to do so.