The peoples of the Russian Empire didn't want to fight and die on the front lines like millions before them. They wanted food which was being distributed towards the soldiers. The peasants had been after the lands of nobility for generations. That was perhaps the principal reason why the revolution didn't peter out, because as soon as anarchy enveloped the cities, instead of fighting to maintain order, the peasants went after the land that they had been hungry for.
Political? What would be a purely political cause instead of an economic one? The two are so intertwined. Perhaps a simple desire for democratic rule? That becomes problematic, for while a portion of the Russian peoples did want self-rule and a more modern system of government than simple despotism, I don't think they were very many or even a significant force in the Russian Revolution. Otherwise democracy wouldn't have been tossed to the wayside so quickly. No, the reason why the Bolsheviks were able to do away with de facto democratic rule almost from the onset even though they had promised it from the beginning was because people were more motivated by what the Bolsheviks could give them: food, peace, land... rather than motivated by principles and ideology, and were willing to give most everything to gain those three.