US Dollar is not a global currency. It is a native currency of US though being used in many other countries (small ones, not in power to develop their own ones). So possible reason for USD crash will be collapse of USA economy. There maybe many possible reasons for it. And about possible outcomes-
1. Increase in exports (From USA)
2. Decrease in imports (To USA)
3. Fall in GDP and per capita income (of USA)
And the most important one, huge shift in world power. US will be dethroned from the post of Super Power
The first two points will actually increase the value of the dollar
If the US increases exports, their exporters will obviously sell their goods only for the US dollar, so whoever wants to buy these goods will have to find dollars elsewhere, and therefore the value of the dollar could apparently rise with exports growing. Basically the same pertains to imports, though in reverse order, obviously. Less imports would mean less dollars going outside of the US, and thus the value of the dollar would increase once again