What causes town guards to spawn?

[Rinnhart]

I've observed a few raids over on Tichondrius (from both defender and attacker views) and I've been trying to figure out what pattern there is to the spawns of guards.

Basically, it seems like after a raid has been hitting a town for a bit, new groups of guards start to warp in.  This seems to happen in packs (that is, 4-5 will appear at once).  The total number of guards starts to exceed the number of guards in the town.

I've heard two explanations for this so far:
1. Everytime you kill a guard, 2 more will spawn in a bit
2. Killing non-guard NPCs causes more guards to spawn
3. Some combination of the above.

I don't think #1 is right because I'm not seeing exponential growth in guard numbers.  I know there are some special triggers on NPCs (e.g. attacking a windrider is almost always an awful idea).  So #2 might be right, but it's hard to tell.

Anyone have any more info?
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Make sure you're taking out their production structures.