Since you don't have a theory, here's mine (tinfoil hat optional): They all go overseas to countries who's currencies are hyperinflating. In some countries, they won't even accept older designs of U.S. bills, meaning there is a massive demand for new bills. It's similar to how we need to ship all of our nat gas to Europe, so suddenly gas stoves and heaters are a major health hazzard and need to be banned. In Weimar Germany, they had to constantly innovate and invent new printing techniques and equipment in order to keep up with the volume of physical bills required. Now the U.S. is trying to do this for the entire world and can't keep up. Even if everyone in the U.S. completely ditches cash, this could become a serious problem in the future. Keep an eye out for massive funding requests from the Bureau of Printing and Engraving in the near future.