Minecraft wiki entry for Zombie_siege, associated text: For the most part, the zombies that spawn as part of a siege behave in the same manner as normally-spawned zombies; they attack all villagers within sight, and villagers and iron golems react accordingly. However, if the zombies happen to wander away a certain distance (greater than the size of the village) from the center of the village, they return toward the village center if they are not actively pursuing a target. The zombies despawn as usual if the player is far enough away.