A NYT article told readers that investors are worred because China may stop buying and could even start selling US Treasury bonds:
"Bond markets appeared to be further spooked on Wednesday by a report that China’s central bank, which owns $1.2 trillion in United States Treasury bonds, may be poised to slow or even halt its buying of United States debt. China has total reserves of just over $3 trillion."
It later added:
"But there is another interpretation that gets at the simmering tensions between the United States and China over North Korea and trade. 'It is possible too that China wants to signal to its people that it will not keep financing the U.S. when the U.S. is not treating China with respect,' Mr. Setser said." [Brad Setser is a senior fellow at the Council on Foreign Relations.]
While China's decision to stop buying, and possibly start selling US Treasury bonds, is presented as a bad thing in this piece, it is exactly what anyone who had complained about China's currency "manipulation" (e.g. Donald Trump) would want to see. This "manipulation" (which should more accurately be called "management" since it is entirely open) involved China's government buying US government bonds and other assets in order to prop up the dollar against the yuan.
By buying dollar-based assets, instead of selling its dollars in international currency markets, China was increasing the demand for dollars, thereby pushing up its price. If it stops and reverses this process, it will be lowering the value of the dollar relative to the yuan. This will make goods and services in the United States more competitive internationally, thereby reducing the US trade deficit.
Rather than being a hostile gesture toward the United States, this is exactly what Trump claimed he was going to make China do in his campaign. He said that he would a take a tough line with China and make it end its currency management.
It is also worth noting that if the dollar declines in the months ahead it would be the exact opposite of what most economists (including the Trump administration's economists) had predicted as the outcome from the tax cut. They had predicted a flood of foreign investment, which would have the effect of increasing the value of the dollar and the trade deficit.