Some might write it to show a 'naughty' or 'edgy' side of a relationship. But this is wrong. A relationship should be a safe haven. Any form of physical punishment, like spanking, is not acceptable behavior. It can cause emotional and physical harm to the girl. In a positive relationship, partners support and encourage each other, not engage in such inappropriate behavior.
Maybe the girl wanted a partner who could understand her world better. If she was into typically female - dominated activities or interests, she might have thought that by making her boyfriend more like a girl, he would be more empathetic towards her experiences.
It is completely inappropriate for any form of non - consensual or inappropriate physical contact like spanking in a normal context. However, if this is in a role - playing or very specific, consensual game within appropriate boundaries (such as in some forms of BDSM where everything is based on strict consent rules), then it could be part of a particular fantasy. But in general society, spanking as a form of punishment or non - consensual act is unacceptable.
One reason could be to avoid unwanted attention from guys she's not interested in. If she doesn't want to deal with someone's advances, saying she has a boyfriend can be an easy way out.
One reason could be disobedience. For example, if a girl is told not to do something like go into a restricted area and she does it anyway. Another reason might be lying. If she tells false stories to her parents or teachers to get out of trouble.
One reason could be a total breakdown of family order, like if the son has some serious mental issues that make him unable to understand proper behavior. Another could be extreme anger or resentment due to some past trauma within the family that has made him act out in this unacceptable way.
It could be related to past experiences, maybe something you saw or heard that left an impression. Or perhaps it's just a random thought that popped up in your mind.
One reason could be to add a comical element to the story. It might be a way to show the competitiveness between the two characters in a light - hearted and exaggerated way.
One possible reason could be a misunderstanding. Maybe the teen girl misinterpreted something the wife did as a wrong action towards her or someone she cares about. For example, if the wife accidentally knocked over the teen girl's art project that she had been working on for a long time, the girl in her sudden anger might have impulsively spanked the wife.
Maybe the mother lost her temper and couldn't control her emotions at that moment. She might have been very angry about something the girl did immediately.
One reason could be guilt. If a boy has done something wrong that he deeply regrets, he might ask for a spanking as a form of self - punishment. It's like he can't bear the internal guilt and wants to get it over with through an external form of discipline.