That's because it autospread to civs with leaders that have that founded religion as a 'favorite religion'. Those civs must also have Divine Cult or greater as a civic. What I'm wondering though is if there are some loophole ways to declare a national religion when you don't have a civic that enables you to do so. Such loophole methods might be a civ demanding you convert. Or perhaps for those civs that got their favorite religions automatically in their lands - were they allowed to thus immediately adopt it despite being normally unable to based on their civics? This could be possible if the coding for the diplomacy and events aren't taking into consideration the inability of the nation to adopt a religion yet.