fnaf 1: they're possessed by murdered children that I think believe u killed them
fnaf 2: some glitchy security protocols or something
fnaf 3: he's a serial killer
fnaf 4: figures of the main character's nightmares and imagination
fnaf 5: no clue
fnaf 6: even less of a clue
ucn: the main character is in H-E-double hockey sticks
security breach: virus