Because their entire economy is built on exporting cheap crap to the US (and other countries).

They run a huge trade surplus with us. Which means US Dollars are flowing into China.

Their two options are:

Hold cash (US Dollars)

Or

Buy treasuries where they can at least collect some interest.

If they spend their dollars their currency will rise relative to dollars, cutting much of their manufacturing advantage (we can buy cheap shit from any number of countries including our own).

They’re trapped in a box and any President that has balls (ie Trump) can exploit that.

Right now they have a friend in the White House unfortunately so they will act like a bully, but their economy is super fragile when you peak under the hood.