Society as a whole is robust, but all aspects of society are fragile. A small change can cause large effects, like how a beetle walking on a sand dune can cause a little cascade of sand. It's the essence of the conservative viewpoint that the world is chaotic and society is complicated, so making changes is always risky. So an uptick in mass shootings doesn't have to be a complicated reason.
On the whole, mass shootings (by the non-spun definition) are still rare but I do believe they are increasing. Any small perturbation in society can cause a shift this big.
It could simply be removing tobacco. A generation or two ago, nearly everyone used tobacco. Now tobacco has been removed, but not really removed, it's substituted by mass amounts of sugar. Psychological impact of this drastic change, on a societal level? Unknown and unknowable (there's no control group).
It could be the phones. You could say social media, but I think the actual carrying of connected devices 24/4 is a bigger factor in terms of disrupting natural dopamine mechanisms. What if you changed this for an entire society? What would happen? Probably big things. Would you ever know? Nope...no control group.
It could be something environmental. We all know the sad and tragic history of leaded gasoline. We had to poison a whole generation of children until big oil couldn't paper over the research any longer to cover up what had been known for 50 years. It's widely accepted that crime has dropped as a result of removing the lead. If removing lead can drop crime, what other chemicals could increase it? What if it's something more subtle than lead? The science and mechanism of lead was dead simple and obvious, but big oil still covered it up for decades. What if it's something a little less obvious or simple this time? Maybe it's the bottled water.
Maybe it's the continued destruction of the basis of society being Christian morals and strong families. Wow there's a simple theory. It's almost like you could actually test this by checking how many mass shooters come from environments with strong basis in Christianity and strong healthy family ties. Somebody should look into that, there might be a clue there.
I understand the mass shooting phenomenon as a phenomenon of men, especially young men. What has changed for young men in our society (hint: basically everything). What if you completely changed the game for a generation of young men in terms of their prospects for a normal relationship with a woman? What if you convinced a whole generation of women that they didn't need men; what would the men do?
It could be the internet porn. Most anti-porn people approach it from a religious prohibition angle. Everyone else is thinks it's good to be sex-positive and masturbation is healthy because that's what they heard. In the meantime porn use has gone up drastically, age of porn use is down drastically, types of porn consumed have changed drastically, and effects of this on the body and brain are unstudied...there's no control group. My generation were just getting dial-up internet when we were in puberty, so we are the last unimpacted generation no not be exposed to large amounts of internet porn during puberty. I personally fear that a whole generation of young men are not just going to have a hard time getting off in a healthy way, but I'm talking way beyond sex, their brains might end up totally wacked, popping ssris like diabetics need insulin, just to function. I really think that in secular society's desire to be sex positive, they are overlooking how much sex and non-sex brain chemistry are related and how big of a deal this is for potentially everyone. Almost like there was a psychobiological basis for those silly religious prohibitions afterall.