I would say: FED effect.
FED has been increasing interest rates since 2015, but that increase has become too steep this year (as you can see from the image below).
This increase in FED's interest rates leads to a decrease of the price of the bonds in general. And don't forget that these countries have billions and billions of dollars invested in bonds, so they lose a lot of money. This is a logical explanation to me, but there might be more reasons behind it.
I also think that the fear of a stronger dollar against other currencies leads to this type of behaviour

Feel free to resteem this post: I don't charge you any money XD
short-term bonds)