Traditionally, the USD and the CHF are the main safe-havens in times of trouble and the big question then is which currency to sell.
At the moment, the EUR is the easiest currency to sell as Sovereign debt worries are never far from the surface so when traders are looking for a reason to be short, they don’t have to look far. The other currency which is often sold in times of risk-aversion is the AUD but that is still a very expensive short to fund.