Since 2010, ... the net international investment position has plunged by some 50 percent of GDP. This time the valuation effects have worked in reverse: the U.S. dollar has strengthened notably since 2010, and U.S. equity prices have risen much more than foreign equity prices. In other words, the value of foreigners’ investments in the U.S. has risen a lot relative to the value of Americans’ investments abroad.
Thus, one way to look at the fall in US net international investment position is that it's the result of good news--a rising US stock market.
The other important pattern here is that, in general, if you have $100 in debt it will pay a lower return than $100 in equity, basically because the debt is safer than the equity. The US investment in foreign equity is often in the form of "foreign direct investment," where a US firm owns a large enough share of a foreign firm that the US firm has a say in managing the foreign firm (although the US firm may not have complete control over the foreign firm). This means that US investments abroad systematically earn more than foreign investments in the US. Here's a figure from Milesi-Ferretti:
I sometimes say that when it comes to international investment, the US economy is like a company that borrows money at a low interest rate and then invests that money in corporate stock and receives a higher rate of return. There are of course risks to this approach, but it's been this way for the US economy for a long time and there are clearly benefits, too.