Human nature dictates that you aren't going to get many female responses acknowledging that is what happened in their case.
Here is what I'm comfortable telling you in my personal experience, those whose circumstances I am familiar with, and on these boards.
MOST people, regardless of gender who say the words, "I love you but I'm non in love with you." and/or, "I'm not happy." combined with substantial changes in their behavior, losing weight, new wardrobe, change in demeanor or personality, away from the home more, etc. ...
are emotionally invested in someone other than their partner.
So the unfortunate part is, even when someone doesn't dump their spouse for someone else, people will presume that an affair was the case.
In the case of a Walkaway Wife, as others have pointed out, this is usually mid-life, after years of marriage in which the woman has simply withdrawn emotionally. At the time she says she is leaving ... although news to her partner, she has likely had that mind-set for a long period of time. It most certainly isn't a matter of her waking up one day and deciding to pull the plug.
Self-identity seems to play a very significant role, as evidenced by the input from the ladies here.
Women in general are far more likely to initiate divorce than men are. Women Initiate Divorce 66% of The Time: Why Do They Want To Get Married? | Love + Sex - Yahoo! Shine
There are other statistics out there that indicate that in college educated couples, women initiate divorce 90% of the time.
Kind of begs the question, doesn't it?