A lot of confusion may be found here, especially under the 'Determinants of FX Rates' section ...
http://en.wikipedia.org/wiki/Foreign_exchange_marketAs horrifically complex as the FX markets look in the above article the currencies and therefore the nations these pieces of paper represent appear (nominally) to be self contained entities that are valued and speculated upon as such. The previous answers about market forces, flight to safety and the rest are accurate but won't clue you into why you might find dollar currency movements potentially counter intuative.
The layer that is rarely mentioned is the geopolitical/geostrategic nature of the dollar as the world reserve currency as established by Bretton Woods I. What this meant was that most global markets (commodities etc) ended up denominated and traded in dollars, that is, the actual transactions require dollars to complete. This dollar status permits the US many potential trade advantages for example in recycling petro dollars, however it also means that large swings in sometimes non US markets produce supply and demand issues for dollars not directly pertaining to the US economy itself. Another consequence is the so called flight to safety in Tbills as covered by a previous poster, this means that people park their capital in short term treasuries if they wish to abstain from speculating abroad and then reliquify these when re-entering other markets. There are other things but I hope you get the idea.
Also be aware that since the adoption of floating exchange rate mechanisms in the '70s and the wholesale 'finacialisation' of the FX markets every conceivable underhand and nefarious activity (naked short selling to outright fraud) prevalent in the so called free markets (an oxymoron) is present in FX markets. These market activities are driven by motives from the purely speculative (hedge funds) to the wholesale economic subjugation of whole peoples by supra national entities (IMF etc). In other words FX (or any other) market movements may have very little to do with market forces as most people understand them. Can you imagine a dollar CDS cascade/unwind notionally valued in fifteen figures?
Hope this helps a bit.