What I see happen half of the time is that the guardian elemental gets revealed while I am exploring. My scouts drive him off but don't kill him completely. The elemental then retreats through the portal thereby cracking the seal. At that point, that particular world starts to spawn mobs and the start making raids through the portal.
If you kill the guardian instead of allowing him to escape, the seal is still intact and the raids do not come. Presumably, according to the patch notes, the mobs are not yet spawning in this world.
I have seen others report, though I cannot confirm it, that wandering mobs will sometimes crack the seal on a portal also. If that is true, then the best thing to do is make sure you kill the original guardian and then plant one of your units right on top of the portal to prevent strays from going through it.