So, this isn't a case of death hiding and finally showing up, it's deathliness spiking — think of it like smoke, and necromancers being the fire alarm in this case. Sometimes the hell gate cracks a little more open, smoke up, MEOOWWWWWWOWWWWWOWWWW. Or sometimes the hell gate is contained by whatever these good folks are doing down there, and the necromancers can feel less death around.