It could be a result of misinterpreting the characters. Maybe some fans don't fully understand Goku's and Chi - Chi's personalities and their relationship, and end up creating something that is completely out of character like this. Another possibility is that some people are exploring dark or alternative storylines just for the sake of being different, without considering the negative impact it might have on the perception of the original characters.
Some people might create it just for shock value. They know that the original relationship between Goku and Chi - Chi is loving, so by creating something so opposite, they can get a reaction from readers.
Maybe in this fanfiction, Goku realizes that he and Chi - Chi have very different personalities. Chi - Chi is often more concerned with family and a stable life, while Goku is a free - spirited adventurer. This clash of values could lead him to decide not to marry her. For example, Chi - Chi may want Goku to stay at home and help raise Gohan, but Goku is eager to explore new planets and fight powerful enemies.
One reason could be his pursuit of stronger opponents. Goku is always eager to test his strength and find new challenges. In fanfiction, this might lead him to leave Chi - Chi to travel the universe in search of the toughest foes. He might feel that staying at home with his family restricts his growth as a fighter.
I don't support the idea of 'Goku abuses Chi - Chi' in fanfiction. Goku and Chi - Chi are a couple in the Dragon Ball series, and their relationship is based on love and family. Creating such abusive fanfiction goes against the positive image of their relationship in the original work.
One reason could be to explore a completely different side of the characters. Maybe the writer wants to see how the Dragon Ball universe would react to such a scandal. Another reason might be for shock value, to get readers' attention with a very unexpected plot twist.
A third possible plot is that Goku and Chi - Chi meet in a parallel universe where their relationship is completely different. They could explore this new dynamic and learn from each other. Maybe in this universe, Chi - Chi is a powerful warrior herself, and they team up to fight a common enemy, which changes their relationship in their own universe when they return.
Well, their meeting in fanfiction could be really sweet. Maybe Goku finally realizes how much Chi - Chi has been there for him all these years. He might see her in a new light, beyond just being his wife. They could have a deep conversation about their past adventures and how they've grown as individuals.
Perhaps there are some who have a darker side to their creativity and don't fully consider the implications of writing about abuse. But this kind of fanfiction should not be encouraged as it goes against the positive nature that most fanfiction communities strive for.
Some people might create it out of a desire to explore dark or 'what - if' scenarios that are completely opposite to the normal relationship in the original work. It could also be a way for them to experiment with different types of conflict in writing, though it's not a very positive or healthy way.
There could be several reasons for the existence of such fanfiction. Firstly, it might be a form of creative exploration for those who want to test the boundaries of the characters' personalities. They may think that by putting Inuyasha in an 'abusive' role, they can dig deeper into his character flaws. Secondly, it could be an attempt to create a contrast. Since the original relationship between Inuyasha and Kagome is mostly positive, showing an abusive side can create a sharp contrast. However, this kind of fanfiction often goes against the general perception and liking of the majority of fans.
In a fanfiction like this, perhaps Bardock is on a mission in the afterlife and it leads him to cross paths with Goku and Chi - Chi. Bardock could test Chi - Chi's strength and worthiness to be with his son. Chi - Chi would rise to the challenge, showing her determination. Goku would be in the middle, trying to make sure his father respects his wife. And in the end, Bardock could give his blessing to their relationship, seeing how happy they are together and how Chi - Chi has made Goku a better person in many ways.