It was thought that with the passing of the Season of the Witch, the invasions would cease, and indeed they have; however, recent arcane communications have confirmed that the players can cause an invasion to take place, in a similar way to that in which Rikti invasions are triggered by the completion of a Lady Grey Task Force. This is excellent news for badge collectors, as the chance to earn badges for zombie defeats now won't be limited to a short time window.
For some reason, when an event in CoX is player-triggered (or believed to be player-triggered) some peculiar theories tend to crop up. For example, several players originally believed Lusca the Giant Octopus was summonable, and you still sometimes come across people who think they can spawn the Giant Monster battle in the Grim Vale.
This one's definitely spawnable, though. The players have been hard at work trying to find out the trigger, and at the time of writing, it seems that turning in Halloween salvage to earn a new costume slot is the number one theory. It's not proven yet, so feel free to join the necromantic researches.