It's a social contagion thing. A place or country that has never had such a thing happen might well go for decades with it continuing to "not happen". But if it

It's a social contagion thing. A place or country that has never had such a thing happen might well go for decades with it continuing to "not happen". But if it starts happening, you get copy cat crimes. People who were apparently too dim to think it up on their own are inspired by the first criminal act, and repeat it. These secondary acts inspire yet more of the same. If the rate gets high enough (likely passed that threshold decades ago), it becomes self-sustaining. It might even be how some riots boil up out of nowhere. Tends to happen alot with teen suicides too.