The trigger for the November plunge was U.S. sanctions on Russian lenders, including state-owned Gazprombank. Up until now, Gazprombank, through which payments for Russian gas were made, was the only major Russian state bank not to have been sanctioned by Washington. The U.S. move means there will be less foreign currency entering Russia as payment for gas exports (until a workaround is found). When the news filtered through, everyone rushed to buy foreign currency and, as Western sanctions have already hollowed out Russian currency markets, any sudden change leads to exchange rate fluctuations.