For the same reason why swiss franc has gained 10% against euro in the last two months. It’s called carry trade unwind.
For the last few years, many big financial institutions had large sums of money in “carry trade” – borrowing money in low-yield currencies (yen, swiss franc) and using it to buy bonds denominated in high-yield currencies (australian dollar, new zealand dollar). This process is being reversed on the massive scale. One theory is that some big hedge funds are liquidating their assets.
