I think that there are many causes. As with any event of this magnitude, you can find a great many reasons why it happened or what helped to trigger it. Yet, I would think that Russia's failures in World War I helped to really make the case for revolution in Russia. The Tsar had been seen as either highly ineffective or downright oppressive of his people, depending on one's point of view. The entrance into World War I was his last ditch attempt to rally public opinion within the guise of war and the nationalism that follows it. As Russian casualties mounted, the perception came to be that the war was something waged by the Tsar and suffered by the poor and the downtrodden that were forced to fight in it. The mounting casualties only proved the point that the Tsar's opponents were right: Either he didn't care or does not want to care about the vast majority of Russian citizens.
The February Revolution in March 1917 was the Provisional Government's attempt to establish order, post Tsar Nicholas. They brokered the agreement with the Bolsheviks for power sharing in the wake of the Tsar's removal. The civil war that ensued between the "Red" Bolsheviks and the "White" provisional government ended up going towards the former because of its enjoyment of a broader base of support in the cities and rural areas. The October Revolution, then, resulted because the Bolsheviks were able to enjoy the support of the public who became disenchanted with the Provisional Government that followed the Tsar.
In the end, I think that the pain from the war helped to remove the Tsar, and the inability to govern coherently helped to move the Provisional Government, which paved the way for Lenin and the Bolsheviks.