Why is the Russian rouble falling?

 From the Economist:  

Russia’s rouble slumped to a 16-month low of 100 against the dollar, having lost around 25% of its value this year. Deteriorating foreign trade conditions and an escalation in military spending have accelerated its fall in recent weeks. The currency was already suffering from western sanctions and European countries’ diversification away from Russian energy supplies.

The exchange rate is the "price" of the rouble which is set in a "market" where quantity demanded = quantity supplied. If the price falls it must be because 

• Demand has fallen (Europeans who sell € (euros) to buy ₽ (roubles) to buy Russian Goods or invest in Russia).   In this case, European sanctions has reduced EU demand for Rusian Gas.  
• Supply has increased (Russians who sell ₽ to buy € to buy EU goods or invest in EU).  The article mentions increased military spending which could represent and increase in demand for €.  More likely, though the article doesn't say this, Russians could be trying to invest in the EU or US to get their money out of Russia because they anticipate a depreciation in the ₽

Note that the Economist talks about depreciation against the $, but we are using € instead of $ because the EU is a bigger trading partner with Russia than is the US.  The depreciation against the € would track the depreciation against the $ if the exchange rate between the EU and US is stable.

UPDATE:  Russia Hikes Rates To 12% In Emergency Move To Halt Rouble's Collapse

By hiking rates, Russia hopes to make investing in Russia more profitable, so that investors will sell € to buy ₽ to invest in Russia, which makes the ₽ appreciate