As others already mentioned, you always trade currencies in pairs. By breaking the peg anyone on the wrong side of the pair lost. Since currencies move very little, most currency trades are highly leveraged. A 20% move in a day basically wiped out entire fortunes.

The other people who are exposed are those who took out swiss franc mortgages from other countries[1]. I'm amazed this was even allowed, but up until now the franc was stable and the rates low so it made some sense.

[1] http://www.economist.com/news/europe/21639760-poles-were-slo...