The end of the Golden Age of man was marked by massive world war. A war larger and more destructive than any that had come before. With the use of super-men, machines of war, giant robots, laser weapons, particle cannons, millions upon millions were killed.
With the killing of a living creature, it's PPE is released to the environment. Normally, this energy will dissipate and maybe have minor lingering effects (poltergeist, ghosts, strange feelings when in a location), or typically no lingering effects at all.
When humans were killed in massive numbers all across the world, all their PPE released to the environment would coalesce in Ley Lines. When enough PPE was released, the Ley Lines tore open Rifts, letting loose monsterous horrors from the beyond. These Dimensional Beings (commonly referred to as D-Bees) entered a world at war, and had no qualms about killing these violent creatures infesting this wonderous mystical place.