There are portals high up in the sky, directly above the infamous site in Munteen.
The madness in Elop Portas started the moment myself and @Scratchnwiff opened a portal at build height (256) above the central part of the city.
The mobs also seem to be spawning in a large area around the portal, so I assume it’s portals causing this anomaly?
Edit; the mobs also died down when I closed the portal, and came back in droves after reopening. The portal is still open at the moment - so if people want to witness this mayhem and help us keep this invasion under control; be our guest!