If you read “Population Density and Social Pathology”, John Calhoun, I think it was Nov, 1970 Sci Am you will find out.
The scarier parts are in the near future - infanticide, cannibalism, even worse than that...
Wish it were that simple, but that doesn’t explain it going into places of moderate to low population density.
(I’d think the reason for prevalence in big cities is simply the access to large enough numbers of like minded people within a convenient travel distance. But as modern modes of travel wipe away such barriers, the big city isn’t even necessary for this stuff to thrive anymore.)