Foreigners have picked up an appetite for US debt. Non-financial foreign companies have borrowed over $9T in US dollars. Much of this money has been borrowed by emerging markets, especially those like Brazil South Africa and Turkey who use dollars to finance their current-account gaps. Because interest rates are so low those holding US dollars have sought out markets where they can hope to make a better return. But debts taken on in dollars must be paid back in dollars and many such foreign firms have meager dollar earnings. Asian firms seem in a better position to repay their US denominated debt because their percentage of debt matches up with their dollar earnings. China is the big exception; it holds large US debt and it is concentrated at several large debtors. Were there defaults, large issues are unlikely, because the banks that have made the loans have received better regulation than in the past.
Source: The mighty dollar: Feeling green