I’ve wondered this myself. Part of it may be (as some here have said) that we are seeing a selective bias in the way these things are (or are not) being reported (surprise! surprise!)
But I also wonder if part of this is owing to the constant Doom! Doom! Crisis! Crisis! “theme” to which we have been subjected by our government and news media. I suspect that there is always a small but dangerous fraction of the population who can be easily “pushed over the edge”.
There may also be an element of “heightened expectations” to this. My wife, who works in Latin America, has noticed that often after a leftist gets elected president in one of those countries, the crime rate goes up. I guess it finally dawns on these people that there is no hope for the politicians’ promises being fulfilled, so they decide to take matters into their own hands.
“The whole aim of practical politics is to keep the populace alarmed (and hence clamorous to be led to safety) by menacing it with an endless series of hobgoblins, all of them imaginary.”
-H. L. Mencken