Gosh, have we in the West forgotten so much of our history already?
Much of the gold was moved out of Europe prior to and during WWI. Partly out of fears of German aggression b ut also because throughout the war, European countries bought massive amounts of materiel from America, which increased the flow of money out of European and into American coffers.
But gold itself is very heavy and thus cumbersome to transport. It doesn't make sense to run transatlantic gold shipments every time a few bars change hands. So central banks tended to keep the gold more or less in one big vault and just sort of move the bricks between one country's pile and another. So there would be, in essence, a large stack of gold in the US Federal Reserve, and that gold belonged to France, or whomever. When the war ended, European countries were devastated and needed goods from somewhere that didn't have depleted industrial capacity and whose farmlands weren't littered with UXO.
This situation more or less cemented America's position as the world's creditor, and so it simply was more convenient to keep a large portion of one's gold reserves in the country with whom you were doing a lot of business. For some countries, repatriating gold is a matter of sovereignty, and e.g. France started repatriating its gold reserves in the 1960s.
Now that Americans have decided they like being a military, economic, and cultural superpower less than they like electing a dementia-ridden child rapist, other countries have correctly realized that America's decline cannot be stopped, only slowed. After all, it's Americans themselves who are the root of the problem, and Trump is but a symptom. This instability is not going to stop when he leaves office. Better get your gold off the sinking ship, but do it slowly so as to not accelerate the sinking before you can get it all out.
Re: the gold situation after WWI, if you're interested I encourage you to read Liaquat Ahamed's excellent Lords of Finance, which won the Pulitzer, IIRC.