Could you link to a source? This is interesting to me but I'd like to know more about their method for arriving at those conclusions. I wasn't able to find the article/study you're referencing.
Thresholds of violence: How school shootings catch on, by Malcolm Gladwell https://www.newyorker.com/magazine/2015/10/19/thresholds-of-...
Gladwell interviewed about the article on National Public Radio’s Morning Edition: https://www.npr.org/2015/10/13/448182623/how-riots-may-help-...
I googled that query because I remember hearing it from the On the media podcast.