Japan has been selling US treasuries

Here's the chart:

source

Their holdings are lower than they've been for five years.

They're selling US Treasuries to obtain liquid dollars, which they use to buy Yen and prevent it weakening even more.

As long as the markets keep attacking the yen, the Japanese will continue selling Treasuries. This then makes US bond yields spike, which US mortgages more expensive, as well as making it more expensive for the US govt to borrow.