Emerging nations own a large number of US treasury bonds, as well as bonds from the eurozone, as a byproduct of them attempting to manage their exchange rates. When an emerging nation believes its currency is too strong, its central bank will print its own currency, and use that to buy the US dollar. It then takes dollars and buys US treasury bonds. This effectively inflates its own currency, making it weaker against the dollar. This also causes inflation. The longer it does this, the more US treasury bonds it collects. When money leaves an emerging market nation, its currency will weaken; more money leaving means more rapid depreciation of the currency. If the central bank doesn't want the currency to be too weak, it must then sell those US treasury bonds, and use the dollars to buy back its own currency - strengthening its currency and causing deflation. It can do this as long as it has US treasury bonds to sell.
So why do we care? Currently, emerging nations in total hold almost 6 trillion dollars worth of reserves - most in dollars, some in euros, the rest in various currencies. China owns 60% of that, with the other 40% spread among the rest. When they decide to intervene to manage their currencies, that affects the market for core nation bonds.
Recently, eurozone core nation bonds along with Japanese bonds and US treasury bonds all declined - which means their yields rose. No inflation was in sight, the economy was not improving, and there were no issues of sovereign default.
The USD tends to rise vs commodity currencies when commodity prices fall. Next chart compares USD vs a bunch of commodity currencies, with the index value of 100 starting in 2007.
Emerging market/asia chart compares USD vs a group of emerging market currencies in Asia, with the index value of 100 starting in 2007.
Here are some individual currency charts. Moves "up" in these charts reflect capital leaving the country.