Even though the above is not fair or necessary, she does this because it's a lot easier to be self righteously angry, shifting all blame to him, rather than go through the painful, yet honest process of mourning the death of a relationship where fault can't really be placed fully on either side.
She doesn't tell the man until she has fully cut herself off emotionally, reversing her feelings for him, and has made most preparations to divorce.
The man may have noticed her feelings cooling to him, but she has deliberately hidden her deeper change, and her future agenda.
So from the guy's point of view, one day a loving wife turns 180 degrees into absolutely hating him, acting in spiteful, psychotic ways he never would have imagined her capable of. It's like a switch going from on to off.
As she has pre-justified it all to herself, demonizing him in her mind, she has no hesitation in acting in ruthless and hateful ways towards him from the moment she declares her intention to divorce.
It is almost always a shock for men to discover this side to women, as for most men this is an unimaginable way to think and behave.
The way it really works is that women do what they want (for any irrational reason) and then rationalize why they did it later. There's no point looking for rational explanations.
If you are trapped with a modern American woman like that, then preparing for divorce and divorcing your wife is your only way out.
You need to assume that your relationship with your wife during divorce will be adversarial. She may start lying and acting in bad faith. Your wife will be influenced by her friends, co-workers, daytime talk-shows, her own divorce attorney, and other sources. She will be made feel like she will be betraying all the present and future generations of women unless she uses all the tricks in her disposal and takes all your money and punishes you.
She will be told by everyone that she is the victim and you are the scumbag, and that she deserves to take you to the cleaners and punish you for the rest of your life.
Many divorce attorneys routinely recommend women to start divorce with false abuse accusation against the husband. That way they can immediately gain the upperhand and the husband is put in the defensive position where he has to spend time and money trying to prove he is not an abuser scumbag instead of trying to get a fair divorce. False accusations by women are becoming standard procedures in divorce courts.
Things you may get falsely accused of in divorce court:
1. Hiding money
2. Emotionally and/or physically abusing wife
3. Emotionally and/or physically and/or sexually abusing children
You can expect a court order barring you access to your house and/or seeing children based on abuse