one of the advantages dollar has over most other currencies is that it's an international currency so when they print more of it to pay back their loans, they don't have to worry about inflation as much as other countries do, because the money created is being divided among most of the world population and not just the US population
but now we have another international currency, the euro, so the new mentality is, if there are two why can't there be three or four
so they are basically declaring an all out financial war against the US, they stop holding money in USDs, that money will circulate back into the US and other countries holding USDs, which is inflationary and undermines the value of the USD