I consider what my H is going through is a major depressive crisis at mid-life. A perfect storm scenario.
I agree KayDee and I also feel it is related to Developmental Stages of the Life Span (Erik Erickson) with a much stronger response than someone who is not in crisis. The perfect storm as you say, fear of getting older or dying, children leaving, loss of important people in their lives, biochemical changes, stress, spiritual crisis...all culminating in a "break" of sorts.
I remember back when I was a teenager and not a very nice person to my parents..no reason, they were very good parents....I remember when my daughter was 16...wanted to give her away. Both my daughter and I went through this stage and then i ended and we became "normal" again. It's a small example perhaps of a similar type of thing.
I have empathy for their "depression" and the stories I have read of MLCers who do write about their experience makes me very glad that I am not the one in crisis.
Yes, RCR did base some of her work on Jim Conway's Men in Midlife crisis. She also studied various other psychology theories, lived through it herself and accumulated years of studying other stories of MLCer's via what has been posted here.
The damage done to the LBSer is significant and so both MLCer and LBSer are going to be greatly changed by this.