The last time I went down to the dungeon I wondered where so many undead came from. Someone said that Magick-Users had put them there.
At first, that doesn’t seem to make sense. But, as Robert Macfarlane describes it, the underworld is the “repository of nuclear waste sites and burial chambers, both a dumping ground and the portal into otherworldly realms.” In the fantasy (or weird fantasy) worlds of OSR games, there is no nuclear waste, but what we might call “magic waste.”
When a Magick-User cast a spell, hazardous waste remains. Perhaps the ingredients (or components) of its magick, even if they become unusable ashes, retain part of the magical properties that have disturbed the stability of nature by producing their effect*.
A Magick-User does not want that waste to represent a danger to him or, probably, to others (although almost all of them are misanthropes, not all of them actively pursue the destruction of humanity), so the best option is to bury them; this sepulchre is not perfect, but when the harmful effects begin to take place, he will no longer be alive or, if he is, he will take care of the matter.
Many years later, or centuries, when a group of adventurers descend in search of treasures, artefacts and relics, in addition to undead, the place will be plagued by anomalous entities from the deep past and outer regions, which will have been awakened or attracted by the residual energies of magick.
Among the most common anomalies is spatial distortion, which explains why a simple and mundane drainage system or an old underground vault have become the labyrinths of often unconnected or inconsequential corridors and rooms we commonly know as dungeons.
Of course, some very sick people, invaded by the disease of logic (in a game of magic and goblins!), will not find this explanation satisfactory. For them I have no better answer than to suggest that they make an appointment with the Psychoanalyst to keep that OCD at bay.
*This means that magick and its effects cause the sensation of something that should not be, but is. Magick, then, is not an anomaly; it is our conceptions of the nature of reality that are inadequate.