clearly, I don’t know.
I have guesses as to what might have contributed to the problem:
the main stream media (now, people can get worldwide fame in a single afternoon);
parents not strictly controlling their firearms and then their kids deciding to take out their frustrations so the world can see;
higher capacity guns allowing for more effect;
lack of closeness (love and caring) of family members and neighbors, so little/no feeling of loss when the shooters kill others;
shooters get stressed out (thus becoming shooters), and look for ways to take out their frustrations - other shooters chose the killing path, so they mimic them;
and the list goes on.
I know, I know - this list kind of sounds anti gun. Sorry about that, not my intention. But, these are my guesses. For sure, the kids getting unsupervised access to guns - bad!