I would associate it with the stress level, pure and simple. In earlier, simpler times the stress level was generally lower, and/or the people had better coping skills. Given enough stress, everyone would snap.
I actually experienced a case a few years back when I was teaching. Had a very attractive, bright, engaging, hard-working Freshman girl in a spring semester course, then had her again in another course in the fall. Over the summer she had gained 40 pounds, no longer washed her hair, wore baggy clothes and, I noticed, had begun "cutting" herself.
I immediately informed the staff psychologist who told me that the girl had "had an unfortunate romantic encounter over the summer."
Now the normal response when someone treats you badly is to get angry AT THEM! Unfortunately, today in the land of tolerance, multi-culti, "why can't we all just get along" you are supposed to feel that somehow it is YOUR FAULT that someone else treated you badly.
Insanity.