1. Large US Debt
2. Fears of intrinsic inflation and eventual Interest rate hikes
4. Trump's China trade war . His Adminstration might be having a field day . Drive the USD as a low as possible but claim you want a stronger USD and that will hurt China trade very much without firing any missile . US goods will be cheaper to buy and that will eliminate most of the trade deficit with China. There is one problem though. China's Yuan is pegged to the USD and China is the Largest Debt holder of US Bonds and Treasuries .
Its a Catch 22 for USD , i think i will make my USD cake and eat it too