Hey there! This happened on my first run and I read somewhere online that the Germans have to push deep into Russia for it to happen. For me it was when they were a few tiles away from Moscow on my second run. They also got stamped by Russia so that ruined my historical run.

In order to do this, I set Germanys power pre-game to max. That way they had the equipment to make it in. My MISTAKE - Was sending Russia 15% of my materials a month - I'm pretty sure American fire power ended the soviet revolution for me

In my game, I pushed hard into Russia which triggered the revolution. The Tzar dies and Lenin takes over as leader of the Soviets. you get exiled, but can easily sign non-aggression pact with Soviets. a few years later another Russian civil war begins between Whites and Reds. Be careful about the Baltics, if you wish to go for Holy Roman Emperor you must own them, do not puppet them or your screwed.