It's heavily implied that after the all staff meeting massacre (likely caused by Glam Endos), that the human staff were stuffed into, and thus, would possess Staff Bots. Hence, why they would be all over the Endo Nursery, where Endos are trained to be violent by someone. (+)

Not to mention the Glamrock Endos' thematic association with Nightmare, like the Staff Bots. And the fact that the Staff Bots recreate GGGL, which is about souls being given new life within new animatronic vessels. (+)