In the 1920's Ivo Shandor was trying to bring about the end of the world and apparently, at that time: failed.
Spengler: It's not the girl, Peter, it's the building. Something terrible is about to enter our world, and this building is obviously the door. The architect's name was Ivo Shandor. I found it in Tobin's Spirit Guide. He was also a doctor. Performed a lot of unnecessary surgery. And then in 1920, he started a secret society. After the First World War, Shandor decided that society was too sick to survive. And he wasn't alone; he had close to a thousand followers when he died. They conducted rituals up on the roof, bizarre rituals intended to bring about the end of the world, and now it looks like it may actually happen!
Fast-forward to 1984 - there is clearly some measure of ghostly activity already in previous years because the three ghostbusters are already interested and investigating how to catch and contain ghosts, however, it is only because of an increase in ghostly activity that they are able to finally do so (thanks to the readings they were able to get from the library incident).
Why, in 1984, was there a sudden uptick in spectral phenomenon, specifically the possession of Dana Barrett and the opening of "Spook Central" - what caused/instigated these events?