@Richman
Right off the bat, I can only think of a few reasons binary stars could collide; they're so close tidal effects drain the orbit(however if one star is draining the other's envelope there's a high change the drained star may instead get to a higher orbit(smaller mass same kinetic energy equals up) or at least counteract the decay), there's enough gas floating or the other star has an extended envelope which causes the orbit to decay through friction, the orbit decays through gravitational waves(though that is more of an issue for heavy things like black holes), some outside mass like a third star disturbs the system(not easily, an orbit takes a lot of energy to change up or down, especially for a big thing like a star, much more likely the star may be flung out of the system...)
Though, I haven't dabbled in astronomy at all aside from picking tidbits from here and there, so take those suggestions with a grain of salt...