Russia was a factor in the outbreak of World War I, but it would be something of a reach to label it as the "cause" of the war. Germany faced bitterness from France because of the humiliation suffered by France during the Franco-Prussian War. German diplomats had known for some time that war with France was eventually inevitable. At the same time, Russia had obligated itself to protect Slavic nations such as Serbia. Russia's ability to mobilize a huge army (over one million men) represented a threat to Germany; should it mobilize, Germany would be forced to fight a war on two fronts. The famous Schlieffen Plan had been developed to deal with just such a situation. After Austria declared war on Serbia, Russia ordered a general mobilization. Germany saw this as a threat to itself, and issued an ultimatum demanding that Russia cease mobilization. When Russia refused to do so, Germany declared war.
It is therefore safe to say that Russia was an important player in the outbreak of the war; but not necessarily a cause of the war.