Long term yields falling while the economy is improving is an odd paradox that also occured in the US in the 80s and 90s. Both times there was disinflationary pressure around the world, just like there is today in developed markets. The US is more interesting than other bond markets because the yield curve tells you a story about what’s going on outside the US and not just within. Even though the economy is improving in the US, global growth is still a big concern, which is why global investors are sending their money to the US. When yields were hovering around 4% at the end of 2013 with inflation at about 1.5%, real yields became pretty attractive, especialy for such a highlight rated asset like US govt bonds. So buying up US govt debt pushed long term yields lower (flattening the yield curve) while boosting the US dollar. This is just a cliff notes version of what’s going on, but it might help with your question.