It could be that in the fictional world, there is some sort of social experiment or strange cult - like ideology at play. The mother might be following the rules of this abnormal ideology which dictates that all children should go through the same physical experiences regardless of gender. This would be a very extreme and dystopian - like scenario.
Perhaps the mother in the story has lost a daughter and is trying to relive that experience through her son in a very wrong way. She might be so grief - stricken that she is blurring the lines between her son and the daughter she lost, and as a result, she is making him wear a training bra as part of her confused attempt to hold onto the past.
One possible reason could be that the mother is suffering from some sort of mental disorder that makes her unable to distinguish between male and female appropriate clothing. Maybe she has a delusion where she thinks her son needs the same support as a girl going through puberty.
In a fictional story, while it's a very odd scenario, it might be used to make a statement. Maybe the mother in the story is living in a very distorted world view where she doesn't understand the difference between genders properly. But such a plot would shock most readers as it defies our common understanding of how parents should act and how boys and girls are different in terms of their physical development.
One possible reason could be the mother's own unfulfilled dreams of having a daughter. Maybe she always wanted a girl and in an extreme way, she tries to transform her son into what she envisioned a girl to be.
Perhaps she's just at home and wants to be as free and comfortable as possible. Bras can sometimes be restrictive and uncomfortable, especially if she has been wearing one all day. It could also be that she's in a private enough setting where she doesn't need to worry about the traditional norms of always wearing a bra.
One possible reason could be stress. Sometimes people in the ABDL community use regression as a way to escape the daily stressors of adult life, and the mother in the story might be doing the same.
It could also be to teach the son about empathy. By experiencing what it's like to be dressed as a girl, he might gain a better understanding of how girls might feel in certain situations. For instance, how they are treated differently in terms of clothing expectations.
One possible reason could be psychological. As she grows up, she might start to imitate her mother unconsciously. For example, she may adopt her mother's mannerisms, like the way her mother talks or gestures. Another reason could be environmental. If she is constantly surrounded by her mother's influence, such as living in the same house with all her mother's things around, she may gradually transform in some ways. Maybe she starts to take on her mother's role in the family, like taking care of younger siblings just as her mother did.
One possible reason could be for fun. Maybe she just wants to see how cute her son would look in girl's clothes for a laugh or a unique photo. Another reason might be that she doesn't have a strong sense of gender - specific clothing. She might think that clothes are just clothes and doesn't see a big problem in dressing her son in girl - like outfits.
Perhaps the mother is facing some personal issues or stress and is unable to handle her son's misbehavior herself. So she resorts to having other women spank him. It could also be that she wants to show her son that his actions have consequences not only in front of her but also in front of other authority figures. However, these are not valid justifications as it can harm the child both physically and emotionally.
One reason could be for fun. Maybe during family gatherings or festivals, it was a form of entertainment to dress the son as a girl. It would make everyone laugh and create a lively atmosphere.
In some cases, it might not be that common but can happen. One reason could be for fun or as a form of play. Maybe the mother wants to have a laugh or create some cute pictures. Another reason could be that she doesn't really see a big difference in dressing a boy or a girl for certain occasions, like at home just for a bit of family amusement.