Answer:
There were several events that could be termed revolutions in Russia. The two most commonly referred to are those of around 1905 - a general revolution and period of unrest - and of 1917 - the Bolshevik revolution and overthrowing of the tsardom.
Both shared the same causes: discontentment with the tsardom, poverty of the masses, lack of control over the government by most, and a growing need for change to the underlying system in power in Russia. Explaining every cause in detail would take many essays and lectures, so see the related links for more detail on each revolution or enrol in a Russian history course.
Many causes:
war, extreme poverty, reversal of progress in the last 20 years, amassing capital in foreign banks, high concentration of proletariat in major cities, oppression of minorities, weak tzar Nicolas II.