The Russian Empire was comprised of any different national groups, most of whom did not want to be part of the empire. So when the Revolutions occurred several groups took advantage of the chaos (and in some cases aided by the Germans; the Ukrainian states and Belarus being a couple prominent examples) and declared independence. This is also why so many smaller groups declared states: in the turmoil of the revolution a multitude of groups were formed espousing a variety of ideologies (though often leaning towards socialism of some sort). As time went on the stronger groups consolidated their power and absorbed the weaker states, and seeing how the Bolsheviks were one of the strongest and most organised they were able to quickly reabsorb most of the breakaway regions.