The dollar, surprising most, strengthened at the height of the financial crisis last winer, because during times of extreme uncertainties the dollar was a "safe heaven".
"Better the devil you know than the one you don't" was driving the currency market. Liquidity is also important. The easy with which you can trade large volumes is at a premium when you want to sell.
Now, the world is a lot less unstable than a year ago. The dollar is no longer the currency of "safe haven". When one looks at the massive amount of liquidity the Fed has injected into helping Wall Street and secondarily Main Street, the world is expecting a larger than desirable amount of dollar sloshing through the global system.
On top it appears the Fed will maintain a zero interest rate policy. Any wonder the dollar will continue to fall?
The FT article here keeps us updated on the dollar malaise.