The causes were social unrest brought about by ineqaulity and political visionaries. The two are not a good combination if you don't want change. The revolution was an inevitable result. The people were ready for change and fed up.
In considering the causes of the revolution, one should also consider the detachment of Nicholas II from the needs of his family and perhaps the influence of Grigori Rasputin, who encouraged Nicholas to deal with the peasants with an iron fist. Nicholas is normally considered a good man at heart; but a poor leader, who had no connection with the common people. While they suffered, he lived in blissful ignorance of their condition. World War I is not totally ancillary. Nicholas took it upon himself to lead the troops at the front to "inspire" them; even though he had no military experience. The fact that troops were sent into the war with no weapons, but rather told to pick up the weapon of the first man who fell was also a factor in the discontent which led to the Revolution.
To find another answer to a contributing cause of the Russian Revolution, you may wish to examine who had regular raids made upon them by the Czar's army, and if Marx had anything in common with them.
I really think that I would look deeper. To me, WWI is more of a proximate cause. I'm more inclined to look at the more fundamental causes of things. I would argue that the Russian monarchy's inability to modernize the country and to give its people a modicum of economic and political progress is what doomed them. WWI was just a catalyst. I think that the very poor economic conditions and the lack of any serious opportunity for political input was what caused the people to be willing and even eager to rebel.
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.
As i thought. It is good to get other expert opinions. I would also link the man Tsar Nicholas was and his "divine right to rule" in terms of involvement in WW1. This is all ultimatley led to discontent within the population of Russia, which the Bolsheviks exploited. Your views?