I think that one can see similar causes in both Russian Revolutions.  The Czar's fundamental lack of leadership helped to feed the initial revolution that overthrew him.  The entrance into the First World War, Russia's disastrous involvement within it, and the perception that the Czar simply failed to respond to the needs of his people helped to make for a compelling narrative that justified overthrowing him.  In terms of accounting for the cause of the second revolution, I think that much credit has to go to Lenin in trying to force a Marxist theory into a setting where it did not apply, but to a population which it proved to be persuasive.  Lenin was able to construct the narrative of power struggle in the provisional government throughout Russia as one in which the proletariat needed to unite against their oppressive owners.  In this light, Lenin was able to develop much in way of legitimacy amongst the people and in doing so, guarantee his own ascension to power.  Lenin was able to crystallize the movement in a clear and focused manned that the leaders of the Provisional Government had failed to do.  Through this, Lenin was able to secure the base of support that would move he and his band of Bolsheviks into a position of power that would come to define modern Russia.