Caused by an eclipse? So I've been re-reading the world of ice and fire, and something popped out to my wife and myself - in addition to parallels of the long night around the world (notably the others, or more accurately, ice/winter, are only mentioned in Westeros but necromancy and dark magic is everywhere), we have three additional pieces of evidence:

One -

There are references to stars, things falling from the sky, and additional celestial bodies at about this same time. One legend even has a second moon in the sky, with dragons coming from them. And, The Bloodstone Emperor (cause of the long night) was a worshiped of an oily black stone and is considered a high priest in the church of "starry wisdom."

Two -

"And the maiden of light turned her back upon the world and the Lion of Night came forth..." And so on. While this could be just referencing darkness, "turning her back" feels very intentionally chosen.

Three -

Staring is in the face (somewhat literally), the Game of Thrones intro. It's the sun. And it gets darker in season 8, or the others, descend.

It's an eclipse guys.