A death is a transit. The soul rises through the Ethereal overlay toward the Astral, and the shadow sinks through the Nethereal toward Malstaris, each riding the thread that bound it to the living. The overlays are corridors, not resting places. A specter forms when that passage is stopped partway through.
Necromancy is thread-work, a form of Deoric, and it pays for every working in life. Cut or clamp a thread while the soul or shadow is still crossing its overlay, and the freed remnant has nowhere left to go. It cannot finish rising or sinking. The body that once held it is gone, so it cannot fall back either. It hangs in the overlay where the cut caught it, stranded. What that stranded remnant then is, and how it comes to serve whoever made it, belongs to the entry on the specter as a creature.