Why did a lot of civilians fight alongside the Nationalist Spanish forces during the Spanish Civil War?

Doing some research on the Spanish Civil War and the many battles that constituted this conflict, I came to notice a thing that I hadn't seen too often in other wars, and that is the sheer amount of civilians that fought in defensive battles alongside the Nationalists. These weren't of the Requete, Falangist, or other armed paramilitary units, instead these were civilians.

For example in the Battles of Belchite, Teruel, Albarracin, a good portion of the Nationalist combatants within these battles were just civilians ( particularly in Albarracin). I haven't seen that much popular resistance in any straightforward conflict before, and I wonder why many people actively took up arms against the Republican Spanish army, despite themselves being civilians.