Why did Russian soldiers abandon the army and take part in the Revolution?

This is a straightforward answer to your question. When the bulk of soldiers, ill-equipped to fight against an enemy with superior weapons, is not backed up by a system of good enough food, clothing and medical aid, their morale is naturlly lowered.

There must have been a feeling that they had been put out to fight and risk losing their lives for a country that did not care enough about them to meet their most basic needs. It isn't at all surprising that many defected and joined the revolution.

Lenin was preaching "Peace, bread, land" and these poor men, suffering cold, hunger, injury and with no hope of a good life when they eventually returned from battle, naturally signed up to this mantra.