I have noticed that outhouses will generate around 20 NPC's at once, often killing a couple in the crush (I use the barricades hurt zombies mod so this might be from them trying to knock down a door, but the doors are often intact)

I use a lot of mods so it might be that, I first noticed it when choosing your own spawn to start the game in an isolated cabin as a park ranger, and suddenly there were tons of people around.

In further games i noticed it happening around any outhouse that I noticed, I did think it might be a scripted story, someone dying of the Knox virus in a bathroom while people comment worry about it, nut the sheer numbers of NPC's make it quite comical as tens of people swarm out of the often 4x4 tile rooms.