http://www.securitydegreehub.com/mass-shootings/

My response: 
I notice one questionable point in the graph, "Armed with" and "Body armor."  It's not an arm.  "Equipped with" would be fair, though I would suggest it be a separate category for support equipment.  Other things in such a category could be battering rams, night vision, etc.  (if present).
I attribute the rise in incidents in part to visibility. With the need for 24 hour news to beat a subject to death in order to maintain ratings, the apparent eternity of information online, and even sites such as your own (Observation, not criticism), mass killers hope for some notoriety in death that they lacked in life, and that visibility is all but guaranteed.
I foresee the trend increasing, along with the choice of weapons.  It will take a cultural shift to reduce it, or improvements in real security, not just security theater.
And of course, cultural, social and economic stress always lead to an increase in visible demonstrations of frustration and pain.