Switzerland's long record of low inflation, political neutrality and large foreign reserves makes the franc a destination for money leaving risk. In stressed markets the franc rises against almost everything, which is exactly when Swiss exporters complain and the Swiss National Bank starts to act.
USD/CHF has historically traded roughly inverse to fiber, since both are dollar pairs with European legs. The most famous single event in the franc's trading history is the swiss-franc-unpeg of January 2015.
Example: EUR/USD rises 0.6% while USD/CHF falls 0.5% on the same session. A trader long both pairs has partly offset their dollar exposure without meaning to.
Related: currency-nickname