As the European crisis worsened the Swiss Franc (CHF) was seen as a safe currency so Europeans attempted to exchange their Euros for Francs. This caused the Franc to appreciate in value, against the Euro, through the summer and fall of 2011.
The Swiss government and Swiss Central Bank (SNB) believe mercantilism will create wealth for the citizens of Switzerland. The Swiss central planners believe that having an abundance of export businesses in Switzerland will create wealth for the citizens of Switzerland as the exporters sell their good and services abroad and pocket a bunch of cash. Thus, the central planners tend to favor exporters. From the article:
At the start of the year, when exporters urged for government and SNB action, ...
The Swiss Central bank continued to intervene in currency markets in 2011 to prevent the CHF from appreciating. This was done to prevent a decrease in export business. Finally after many failed attempts they announced the 1.20 peg in September.
The central planners give little consideration to imports, however, since manufacturers in foreign countries don't vote or contribute to the campaign funds of the central planners in Switzerland. As the CHF strengthened many imported items became very cheap for Swiss citizens. This was of little concern to the central planners.
Currencies are like other goods in a market in that they respond to supply and demand. Their value can change daily or even hourly based on the continually varying demands of people. This can cause the exchange rate to rise and fall against other currencies and goods. Central planners mistakenly believe that the price of certain market items (like currency) should not fluctuate. The believe there is some magical number that will cause the market to operate "better" or "more correctly".
How does the SNB maintain the peg? They maintain the peg by printing Francs and purchasing euros.