Take a deep breath... and enjoy!
Portals seem to have opened in several places but they closed in a day. Perhaps the Daimons didn't have the power to sustain them all at once, perhaps someone managed to defeat them, perhaps it was just an intimidation act, perhaps it is something entirely different. The point is: our characters don't know why it happened, just that it did, and are possibly very scared about it. For all they know they may return at any moment, or this could have been a hoax, or whatever. There are many ways to play it out, and not finding a rational explanation? Yep, it's one of them.