One hunch was that it was contagious. X finds out Y mowed down 10 in a bar. X decides to leverage that tactic to right an assumed wrong.
And, two, media coverage about the alleged shooter X, both by establishment outlets and social, creates the expectation in Y that is a path to fame.
Now, those two hunches have been confirmed by research at Western New Mexico University by Jennifer Johnston, Ph.D.
As David Bixenspan reports in LawNewz, gun violence was found to be contagious and spread by establishment and social media coverage. In the past 15 years, the 24/7 news cycle, along with the addition of so many media platforms, has intensified that kind of coverage. Here is the study.
The recommendation is that members of the media focus on details of the crime and investigation, not the alleged or proven criminal. For example, the public should be informed by experts about theories of motivation and progress in the manhunt. But the alleged or proven-guilty miscreant should not be featured in depth. That creates an anti-hero. And it's the anti-hero, such as television characters Tony Soprano and Walter White, who dominates popular culture.