Wild guesses: Do the necromancers have the goal to "rule the world"? Does the embark selection list them with a red line or "war"? Do their ethics or beliefs forbid warfare?
Aside, I haven't ever experienced a full necromacopalyse, but that may be because I've rarely generated +500 year worlds, and those were always large or with raised settlement cap. In my experience, the necromancer guilds either remain puny, or grow and snowball until they hit another guild, at which point infighting hampers both their war against the living.
My goal is to make snowballing guaranteed essentially, the idea for this project is the creation of a world that is doomed to an apocalypse. Anyways, the embark selection does list them as at war, and the ethics of their civs are pretty war-like. All the ones I checked also desire to rule the world.
My necromancers:
[INTERACTION:PRIMORDIUM_SECRET1]
[I_SOURCE:SECRET]
[IS_NAME:the secrets of primordium]
IS_SPHERE:DEATH
[IS_SECRET_GOAL:IMMORTALITY]
[IS_SECRET:SUPERNATURAL_LEARNING_POSSIBLE]
[IS_SECRET:MUNDANE_RESEARCH_POSSIBLE]
[IS_SECRET:MUNDANE_TEACHING_POSSIBLE]
[IS_SECRET:MUNDANE_RECORDING_POSSIBLE:objects/text/book_instruction.txt:objects/text/secret_primordium.txt]
[I_TARGET:A:CREATURE]
[IT_LOCATION:CONTEXT_CREATURE]
[IT_REQUIRES:MORTAL]
[IT_AFFECTED_CREATURE:HUMAN:ALL]
[I_EFFECT:ADD_SYNDROME]
[IE_TARGET:A]
[IE_IMMEDIATE]
[IE_ARENA_NAME:Necromancer]
[SYNDROME]
[CE_DISPLAY_NAME:NAME:primor:primori:primori:START:0]
[CE_ADD_TAG:NOEXERT:NO_AGING:NO_EAT:NO_DRINK:NO_SLEEP:NO_PHYS_ATT_GAIN:NO_PHYS_ATT_RUST:START:0]
[CE_CHANGE_PERSONALITY:FACET:ASSERTIVENESS:50:FACET:CRUELTY:50:FACET:PRIDE:50:FACET:AMBITION:50:FACET:ANXIETY_PROPENSITY:50:FACET:TRUST:-50:START:0:ABRUPT]
[CE_CAN_DO_INTERACTION:START:0]
[CDI:ADV_NAME:Raise Strong Undead]
[CDI:INTERACTION:UNDEAD_RAISE_STRONG]
[CDI:TARGET:A:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:A:10]
[CDI:VERB:disperse primordium gas:disperses primordium gas:NA]
[CDI:TARGET_VERB:shudder and begin to move:shudders and begins to move]
[CDI:WAIT_PERIOD:100]
[CE_CAN_DO_INTERACTION:START:0]
[CDI:ADV_NAME:Raise Tough Undead]
[CDI:INTERACTION:UNDEAD_RAISE_TOUGH]
[CDI:TARGET:A:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:A:10]
[CDI:VERB:disperse primordium gas:disperses primordium gas:NA]
[CDI:TARGET_VERB:shudder and begin to move:shudders and begins to move]
[CDI:WAIT_PERIOD:100]
[CE_CAN_DO_INTERACTION:START:0]
[CDI:ADV_NAME:Raise Agile Undead]
[CDI:INTERACTION:UNDEAD_RAISE_AGILITY]
[CDI:TARGET:A:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:A:10]
[CDI:VERB:disperse primordium gas:disperses primordium gas:NA]
[CDI:TARGET_VERB:shudder and begin to move:shudders and begins to move]
[CDI:WAIT_PERIOD:100]
[CE_CAN_DO_INTERACTION:START:0]
[CDI:ADV_NAME:Build Reanimate]
[CDI:INTERACTION:RAISE_REANIMATE]
[CDI:TARGET:A:TOUCHABLE]
[CDI:TARGET_RANGE:A:10]
[CDI:VERB:perform surgery on a corpse:performs surgery on a corpse:NA]
[CDI:TARGET_VERB:shudder and begin to move:shudders and begins to move]
[CDI:WAIT_PERIOD:200]
Some more testing revealed something interesting. When I replace everything my necromancers can do with this
[INTERACTION:PRIMORDIUM_SECRET1]
[I_SOURCE:SECRET]
[IS_NAME:the secrets of primordium]
IS_SPHERE:DEATH
[IS_SECRET_GOAL:IMMORTALITY]
[IS_SECRET:SUPERNATURAL_LEARNING_POSSIBLE]
[IS_SECRET:MUNDANE_RESEARCH_POSSIBLE]
[IS_SECRET:MUNDANE_TEACHING_POSSIBLE]
[IS_SECRET:MUNDANE_RECORDING_POSSIBLE:objects/text/book_instruction.txt:objects/text/secret_primordium.txt]
[I_TARGET:A:CREATURE]
[IT_LOCATION:CONTEXT_CREATURE]
[IT_REQUIRES:MORTAL]
[IT_AFFECTED_CREATURE:HUMAN:ALL]
[I_EFFECT:ADD_SYNDROME]
[IE_TARGET:A]
[IE_IMMEDIATE]
[IE_ARENA_NAME:Necromancer]
[SYNDROME]
[CE_DISPLAY_NAME:NAME:primor:primori:primori:START:0]
[CE_ADD_TAG:NOEXERT:NO_AGING:NO_EAT:NO_DRINK:NO_SLEEP:NO_PHYS_ATT_GAIN:NO_PHYS_ATT_RUST:START:0]
[CE_CHANGE_PERSONALITY:FACET:VIOLENT:50:FACET:ASSERTIVENESS:50:FACET:CRUELTY:50:FACET:PRIDE:50:FACET:AMBITION:50:FACET:ANXIETY_PROPENSITY:50:FACET:TRUST:-50:START:0:ABRUPT]
[CE_CAN_DO_INTERACTION:START:0]
[CDI:ADV_NAME:Raise Undead]
[CDI:INTERACTION:UNDEAD_RAISE]
[CDI:TARGET:A:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:A:10]
[CDI:VERB:disperse primordium gas:disperses primordium gas:NA]
[CDI:TARGET_VERB:shudder and begin to move:shudders and begins to move]
[CDI:WAIT_PERIOD:100]
[CE_CAN_DO_INTERACTION:START:0]
[CDI:ADV_NAME:Raise Revenant]
[CDI:INTERACTION:RAISE_REVENANT]
[CDI:TARGET:A:TOUCHABLE]
[CDI:TARGET_RANGE:A:10]
[CDI:VERB:disperse primordium gas:disperses primordium gas:NA]
[CDI:TARGET_VERB:shudder and begin to move:shudders and begins to move]
[CDI:WAIT_PERIOD:200]
[INTERACTION:RAISE_REVENANT]
[I_TARGET:A:CORPSE]
[IT_LOCATION:CONTEXT_ITEM]
[IT_AFFECTED_CLASS:GENERAL_POISON]
[IT_REQUIRES:FIT_FOR_RESURRECTION]
[IT_FORBIDDEN:NOT_LIVING]
[IT_MANUAL_INPUT:corpses]
[IT_CANNOT_HAVE_SYNDROME_CLASS:WERECURSE]
[IT_CANNOT_HAVE_SYNDROME_CLASS:VAMPCURSE]
[IT_CANNOT_HAVE_SYNDROME_CLASS:DISTURBANCE_CURSE]
[IT_CANNOT_HAVE_SYNDROME_CLASS:RAISED_UNDEAD]
[IT_CANNOT_HAVE_SYNDROME_CLASS:RAISED_GHOST]
[IT_CANNOT_HAVE_SYNDROME_CLASS:GHOUL]
[I_EFFECT:RESURRECT]
[IE_TARGET:A]
[IE_IMMEDIATE]
[SYNDROME]
[SYN_CONCENTRATION_ADDED:1000:0]
[SYN_CLASS:RAISED_UNDEAD]
[CE_DISPLAY_TILE:TILE:165:3:0:1:START:0:ABRUPT]
[CE_DISPLAY_NAME:NAME:revenant:revenants:revenant:START:0:ABRUPT]
[CE_PHYS_ATT_CHANGE:STRENGTH:300:0:AGILITY:300:0:TOUGHNESS:300:0:RECUPERATION:1000:0:START:0:ABRUPT]
[CE_ADD_TAG:NO_AGING:NOT_LIVING:STERILE:EXTRAVISION:NOEXERT:NOPAIN:NOBREATHE:NOSTUN:NONAUSEA:NO_DIZZINESS:NO_FEVERS:PARALYZEIMMUNE:NOFEAR:NO_EAT:NO_DRINK:NO_SLEEP:NO_PHYS_ATT_GAIN:NO_PHYS_ATT_RUST:NO_THOUGHT_CENTER_FOR_MOVEMENT:NO_CONNECTIONS_FOR_MOVEMENT:START:0:ABRUPT]
[CE_REMOVE_TAG:HAS_BLOOD:TRANCES:MISCHIEVOUS:START:0:ABRUPT]
I get the zombie apocalypse I want. I have eliminated all other possibilities. Something about this interaction and how it changes a creature produces the zombie war effect. For context, my lieutenant equivalent turns the corpse into a "living" creature, a different creature than it was originally, with the following tags:
[NOT_LIVING]
[SUPERNATURAL]
[NOEXERT]
[NOPAIN]
[NOBREATHE]
[NONAUSEA]
[NO_DIZZINESS]
[NO_FEVERS]
[PARALYZEIMMUNE]
[NO_THOUGHT_CENTER_FOR_MOVEMENT]
[NO_EAT]
[NO_DRINK]
[NO_SLEEP]
So, none of these can be the determining factor.