It is the countries that amassed reserves in the first place that probably have more to worry about. When their central banks began buying foreign assets, they paid for them with newly printed money, thereby increasing the money supply. As they switch from buying foreign assets in order to suppress their currencies to selling those assets in order to prop them up, they will correspondingly shrink the stock of money.

How does buying foreign assets suppress domestic currency, and how does turning around and selling those assets shrink the stock of money?