From opening paragraphs:
The hungry demiplane is an intelligent, living entity that feeds on the pain and suffering of those trapped within. As it feeds, it grows by stealing rooms and other structures from the material plane.
The description of the demiplane herein is not dependant on it having been created in any particular fashion. Therefore, you can tailor the plane's origin to suit your campaign. The following is a list of possibilities:
The plane was willed into existence by a demonic power or an evil god.
The plane splintered off from one of the evil or chaotic outer planes, and has taken on a life of its own.
The plane was created through the foul magic of a powerful mage or priest (who might still inhabit it).
The plane spontaneously generated in the Ethereal.
Regardless of its origins, the plane possesses the power to absorb portions of the material plane. Its first act was to absorb a bugbear lair, whose inhabitants it convinced to become its servants. These bugbears serve the plane by kidnapping people from the material world and bringing them back to the demiplane to torture. In exchange, the unique magic of the plane provides the bugbears a secure stronghold from which to launch raids into the material world.