A lot of the value of the USD is that it's the defacto currency for oil and other markets.

Other countries do business in dollars. This makes the dollar valuable as a trade currency. This means large foreign governments like(and need) to hold large amounts of USD.

The confidence in the USD is dropping very quickly, especially in Europe and Asia. This means countries will move to do business in other currencies, as those currencies will be viewed as more stable(or whatever other reason).