Why are Russians withdrawing money from banks?

From today's WSJ:

In Russian cities, anxious customers started lining up on Sunday in front of A.T.M.s, hoping to withdraw the money they had deposited in banks, fearful it would run out. The panic spread on Monday.

...On Monday it plunged further, with the value of a single ruble dropping to less than 1 cent at one point. As the value of any currency drops, more people will want to get rid of it by exchanging it for one that is not losing value — and that, in turn, causes its value to drop further.

In Russia today, as the purchasing power of the ruble drops sharply, consumers who hold it are finding that they can buy less with their money. In real terms, they become poorer. Such economic instability could stoke popular unhappiness and even unrest.