Why is the rouble collapsing? What are the reasons why the rouble (RUB) is collapsing? Which one is the main one?
The primary source of the recent ruble collapse has almost certainly been the falling international oil price, aggravated by some other features of Russian politics and its economy.

Petroleum products account for over half of Russia's export revenue, and most remaining export revenue comes from other commodities, whose output it cannot easily adjust. When oil prices fall, this lost export revenue must be replaced by some combination of

(1) more (presumably non-oil) exports

(2) fewer imports

(3) a larger net capital inflow.

For the most part, these adjustments will be made in response to an equilbrating decline in Russia's exchange rate. Since Russia's export mix is so commodity-heavy, the short run response of (1) to the exchange rate is very weak; since consumers tend to be sluggish in responding to price changes, (2) is weak in the short run as well. For a modern economy with good access to international capital markets, (3) is the main short-term buffer for shocks like these, but Russia is currently suffering from Western sanctions and was not particularly well-integrated into international capital markets in the first place.

For all these reasons, a very large decline in the exchange rate is needed to induce enough of (1), (2), and (3) to offset the decline in oil export revenue. Meanwhile, skittishness about the political situation in Russia (and its dubious historical record on monetary stability) don't help. 

By the way, the dependence of commodity exporters' exchange rates on commodity prices is a staple of international finance; it has even been used to forecast commodity prices.