Due to the lack of a massive organization, the first [Ukrainian] anarchist and anti-fascist volunteers went to war individually as single fighters, military medics, and volunteers. They tried to form their own squad, but due to lack of knowledge and resources, this attempt was unsuccessful. Some people even joined the Azov battalion and the OUN (Organization of Ukrainian Nationalists). The reasons were mundane: they joined the most accessible troops. Consequently, some people converted to right-wing politics.
Mundane would be refusing to eat because you had just brushed your teeth, not "eh, guess I'll join these Nazis." Not to mention given the fact they live in Ukraine and are supposedly anarchists they'd be aware of all the shelling in East Ukraine going on all those years, and all the Nazi punks in the various ranks of military and police. You know, they'd have been against them before the Russian military operation....