Because a lot of people including institutions like Goldman Sachs and the FX brokers were short-selling the CHF (selling borrowed CHF on the open market hoping to pay it back at a lower price denominated in another currency). Except the trade went against them because it was worth more denominated in other currencies. What I don't know myself though is how they were expecting it fall if it was pegged. Perhaps they announcement coming but thought it would fall in price instead of rise.