This is interesting. I don't know why it's happening. However, this book deserves a mention:
https://en.wikipedia.org/wiki/The_Better_Angels_of_Our_Natur... . It shares statistics on how violence has been decreasing throughout the history of humanity.
as a followup to that (excellent) book, here's Barry Glassner - A Culture of Fear. The Better Angels of Our Nature talks about how violence has always been declining. A Culture of Fear talks about how the rate of that decline has been increasing since the 90s but people actually perceive things as becoming more dangerous rather than less, and attempts to come up with an answer as to why that may be the case.