> What prompted this to happen now?

The State Department expected that they'd be able to pressure China into going along with the Russia sanctions. China did not. Now the State Department feels threatened and is trying to show China who the big dog is.

There's been a string of minor insults going back and forth.

Pelosi went to Taiwan.

China announced the will no longer cooperate with non-citizen extradition requests from the US.

A bunch of other stuff I've forgotten.

This is just the next item in the series.