In vast many cases they were either forcefully conscripted by whomever was holding power in the particular area, or were simply choosing to enter armies because they were very young people who lost everything and did not have many options. Do not also discount that during the post-revolution civil war, food shortages were the norm, and the place to eat is army.
I mean that the side winning a civil war probably needs to have a sufficient degree of public support. If enough people went as far as fighting voluntarily, then there was also a significant number who supported this side more passively.
I am not a historian, but I think the society turned this way, in a sense. It's not like some external force coerced them, when everyone was against it.
Hashek is an example. He was not forcefully conscripted, it was his choice.
I've just recently listened to Kerensky's interview (the chairman of the Provisional Government that was ousted by the October Revolution in 1917). He was unable to gather any forces, no-one defended him.
