One hypothesis is that foreign central banks that were accumulating trillions of dollars finally figured out that they were likely to be holding these reserves for years to come, and could afford to put at least some of the money into medium-term US treasury notes yielding (initially) far higher returns than T-bills.
Una hipótesis es que los bancos centrales extranjeros que estaban acumulando billones de dólares finalmente se dieron cuenta de que era probable que seguirían teniendo esas reservas durante años y que podían situar al menos parte del dinero en bonos a mediano plazo del Tesoro de los Estados Unidos que dieran (inicialmente) dividendos mucho mayores que los de las letras del Tesoro.