See this article - does this imply a lot of people were holding short positions on the currency? Given the well-known upward pressure, why would anyone do this?
The losses affected retail customers and smaller brokerages the most. The one-day price change in the CHF was the largest of any major currency since World War One, according to that BBC article, and this one from Reuters. It wasn't necessary for traders to be holding large short positions to sustain losses. The massive surge in volatility wiped out small investors and their losses were passed on to brokers, who became insolvent and/or out of regulatory capital compliance.
The Swiss National Bank's move was definitely not anticipated. At the height of the Euro crisis in 2011, SNB pledged to keep the CHF pegged to the Euro at 1.20 and just last month told the ECB it would still defend it. SNB finally gave up at the prospect of spending billions more to continue.
It wasn't necessary for lots of small customers to be short to sustain losses. People who do currency trading are rarely just short in one currency pair. The huge volatility surge, due to the CHF move, roiled forex markets in multiple currencies. When everything breaks down, losses get passed to brokers, but this is a very uncommon occurrence. (Non-forex customers might hedge a single position if there were CHF forex exposure, but that wouldn't wipe them out.)
- The euro plunged against the franc, going down by nearly 28% as the SNB news broke.