Unlike stocks, futures, or options, currency trading does not take place on a regulated exchange, and it is not controlled by any central governing body. There are no clearing houses to guarantee trades, and there is no arbitration panel to adjudicate disputes. All members trade with each other based on credit agreements. Essentially, business in the largest, most liquid market in the world depends on nothing more than a metaphorical handshake.
The FX market is different from other markets in other unique ways. Traders who think that the EUR/USD might spiral downward can short the pair at will. There is no uptick rule in FX as there is in stocks. There are also no limits on the size of your position (as there are in futures). Thus, in theory, a trader could sell $100 billion worth of currency if they have sufficient capital.