It's a nice touch that the Ghouls in A Thousand Shapes of Horror spawn in the graveyard. Dream-Quest of Unknown Kadath makes it pretty clear that the "ghouls of the waking world" actually live in the Underworld, and simply cross over from the Dreamlands to feast in waking-world graveyards. So the Ghouls are only here due to random proximity to the Underworld entrance, because they're coming up from the Dreamlands' Underworld for their snack time, unrelated to the Unnamable itself.

Similar to the Web of Dreams overarching story, I suspect that what happens to your house in The Gathering is also probably a Dreamlands/Underworld-merging-with-the-waking-world thing, even if it's never explained in Night of the Zealot.

(Mechanically, it's also a way to make area-of-effect damage more relevant beyond just the swarming mechanic (in fact, probably more so, since most swarms are more easily dealt with via normal attacks/evades).)