In my opinion it's because the chestburster growing inside and the facehugger attached to the host can sense danger. In Alien, the facehugger came off when the humans weren't around, and the burster emerged when the crew was eating, so it may have perceived that as a distraction. In the other films, they may have hatched out in front of armed humans because they had simply waited too long. But in AVP, the hatching was perfect. The bursters emerged all at the same time, and the predalien burster emerged whenever the elder predators had gone away.
~For centuries, human beings have worshipped these beasts as gods, beasts that will forever haunt us, beasts waging an eternal war.....~
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