Well, we stand corrected.
There were two in the decade prior to the 1968 gun control Act.
If you dig a mite, you will find that was relatively few compared to decades prior.
However, mass shootings really increased afterwards.
I'm no psychologist, but people get less rational when they feel the walls closing in, and government in general has become more invasive and totalitarian since.
About Charles Whitman: (same source)
It has been suggested that Whitman's violent impulses, with which he had been struggling for several years, were caused by a tumor found in the white matter above his amygdala upon autopsy.[1]