Maybe they wanted to take out a large hedge fund who has tons of short positions by surprise
Central banks don't endanger their huge holdings and their country's economy for petty revenge like that. No, there must have been a better reason. I just can't figure out what.
Don't underestimated the scale of those hedge funds. Many years ago, Soros took out the wealth of many small country by just shorting their currency. Using leverage, those hedge funds can command a capital much larger than a country's central bank's currency reserve. He only met resistance in HongKong, which is backed by the whole USD reserve of China
I can imagine, many such funds were shorting CHF during the past year since SNB was busy buying all the Euro they can to prevent the CHF from rising, SNB made those funds' short position a risk free ride
If it was some thing much bigger that made SNB's sudden move, it should already happened, but so far nothing