Japan raises money by taking out loans. It does this in the form of bonds: you give us 90 yen now, we give you 100 yen later.
Japan's bonds have a low interest rate. This frustrates Japanese bond holders. They want the 5% rates America is paying. So they are selling their bonds. And when you sell a bond, you get the currency of the country that issued it: yen. Now those people, who were bond holders, are now yen holders. But they don't want yen, they want American bonds. So they exchange the yen i.e. sell it for USA bonds.
And when you sell something, its price falls. Bitcoiners know how that works: when there's lots of sell pressure, bitcoin's price falls. Same with the yen.
So the yen's price is collapsing relative to the US dollar as people keep selling it.