It seems like you have long stretches of time without any mass murders, then a sudden burst of multiple attacks. Examples include the Chinese stabbing sprees from several years ago, and the current shootings in the US. Why might that be? Just copycat incidents, or some other cause?
Is this really the case, or it it like the old “bad things come in threes” idea. Perhaps it all depends where you start or stop counting.
Humans do not have a good sense for random events–clusters happen much more often than we intuitively expect (it’s trivial to distinguish between a list of actual coin flips vs. one generated by a human, simply by measuring the cluster lengths). So clusters that do happen seem surprising to us, when it’s actually just the typical state of affairs.
The massive attention paid to some desperate, demented, depressed creature may be very attractive to others of their ilk, and the easy availability of incredibly lethal weaponry makes it very doable.
Otherwise known as copycat killings… the mall shooting in Oregon may have sparked something is this demented young man.
This is by far the most likely explanation. Clusters of events are the norm, not the exception. A simple way to think about this is to consider lottery numbers. People tend to expect that a random draw will be well distributed, one from the tens, one from the twenties etc. In fact, the set of possible draws containing groups of clustered numbers is far higher, so they occur more often.
It’s the same with any essentially random event. Visible comets appear about every decade on average. It would be very unusual to sight three all ten years apart.