Of course, nobody else knows for sure either.
The scarcity of incidents and even more scarce surviving shooters means samples are small and common factors are numerous. We've got various eggheads rooting around in the brains of the surviving shooters, but we're a long way off from discovering a root cause.
Of course, there's pet theories in abundance. And pet theories breed like tribbles and shit like ferrets.
Which means it's easy to grab onto something you were ambiguous or even hateful about and blame it for the random terror of mass shootings. Guns, prescription drugs, lack of spanking, toxic masculinity, pick your fucking poison.
And the fuck of it is, for all the screaming, the number of deliberate godsdamn liars are relatively miniscule. It's just a handful of folks pulling a theory that makes sense to them out of their ass, and then an expanding cloud of believers repeating various levels of educated guess as fact.
I've been asked repeatedly what causes these. And I have my own theories, just no way to properly slap an "origin: my ass" label before sending it on its merry way. So I'm left with "I dunno, neither does anyone else, unless a breakthrough came in last night that I don't know about."