Well, the cult of Sirius is a religion by itself. So they go around trying to "spread the word".
The problem is, they have a right to beleive in whatever they wish. However, nothing tell you that they may be trying to work for the Alien cause even when they are not taken over by the Aliens.
That's usually my first faction taken over by the Aliens tough, so I often end up attacking them on sight afterward. And if I don't raid them and give them enough time when they are taken, they always end up attacking one of my bases.