>>59021498It started on /x/ where somebody said "make a story about this image" where the classic backrooms photo was posted.
Basically, all it was at the time was an infinitely-sprawling dimly-lit area and that's it. Then somebody else later on at the SCP website decided to add in that you feel "watched", which later turned into a "monster talking you".
Then the whole "liminal" zoomer horror craze happened and now there are entire communities dedicated to writing "floors" of the backroom, like the popular "poolrooms" that have pools everywhere and sprawling water slides but have "a monster" walking around.