I don't have an definitive answer, but generally radical movements (like anarchism) sometimes change the expected spellings to appear more non-traditional/anti-

I don't have an definitive answer, but generally radical movements (like anarchism) sometimes change the expected spellings to appear more non-traditional/anti-establishment. In this case, okupa comes from ocupar, "k" is not used a lot in Spanish so in order to appear even less mainstream, changing the "c" for a "k" makes it stand out more. That'd be my guess at least. Maybe someone could track down a definitive answer.