The social media had saturated the society with the affordability and availability of portable devices.
Combine that with the instantaneous news cycle and it becomes a social contagion that every disturbed kid and young adult wants to become famous by righting the incalculable wrongs committed against him in a very bloody and public way.
And it certainly does not help when you have a media that, in pursuit of it’s anti gun agenda, does not bother informing the public about key details of the mass incidents at schools. For example, the perpetrators of the Columbine tragedy rescheduled their bloody rampage to the day that the Colorado state legislature was to debate (and likely pass) legislation to allow law abiding citizens to obtain concealed carry permits for handguns. Both the young teenagers opposed this law and, IIRC, at least one of them wrote to their state legislator expressing this opposition. And that poor response time on the part of police and other emergency services greatly helped to contribute to the high death toll that took place.
There used to be a thing called the “Social Contract,” unwritten rules about behavior and things you just did not do.
No one robbed trains, till the Reno bros did it, then train robberies became common.
No one robbed a bank in daylight till the James brothers did it. Now bank robberies are still common.
You did not shoot up churches schools or places where people gathered, till someone did it. Now it is common.
Of course, closing down the mental institutions in the 1970s and letting the crazies back out on the street certainly did not help.
About every mass shooter has shown clear signs of mental problems but the common excuse is now “He is just off his medicine” still is the main reason to keep them among society and killing.
Then there are violent video games and revenge movies galore made worse since 1969 and the removal of the Hays Code in movies.