Perhaps Minato was under extreme stress. There could have been a lot of pressure on him to protect the village and his family. And if Kushina did something that he perceived as going against his plans for protection, like trying to take Naruto to a dangerous place in her own way of protecting him, Minato might have snapped and slapped her.
One possible reason could be a misunderstanding. Maybe Minato thought Kushina had done something to endanger Naruto when in fact she hadn't. In the heat of the moment, he lost his temper and slapped her.
It could be due to some external influence. For example, an enemy might have used some kind of genjutsu on Minato, making him see Kushina as a threat. So, without fully realizing what he was doing, he slapped her. This would create a lot of drama in the fanfiction as they would have to deal with the aftermath of such an action.
In a fanfiction, if Minato were to slap Kushina, it would be a huge shock to the Naruto - Kushina relationship. It could potentially cause a rift between Naruto and his father Minato in Naruto's mind. Also, Kushina would likely be deeply hurt and angry, which might lead to her distancing herself from Minato. This event could also change the way Naruto views his family's past and his parents' relationship.
Minato would probably be very proud. He might notice Naruto's use of his own jutsu and be impressed by how Naruto has developed his skills. Kushina would be more emotional, maybe crying at the sight of Naruto growing up so well.
Maybe out of curiosity. They want to see how the fans view Kakashi, who was an important part of their lives. Kakashi was Minato's student and Kushina knew him well too. So they are interested in the different perspectives presented in the fanfiction.
On a more personal level, they could face internal guilt. As Naruto grows up and achieves great things on his own despite their neglect, Minato and Kushina may realize what they've missed out on. This realization could cause them a great deal of internal turmoil and regret for not being there for their son when he needed them.
One possible reason could be a miscommunication in the fanfiction. Maybe some secrets were kept from Naruto about his parents that made him feel betrayed when he found out, thus leading to hatred.
One idea could be that as Minato is dying, he passes on his special jutsu knowledge to Naruto in a final moment of father - son connection. Kushina, on the other hand, tries to comfort Naruto while also mourning Minato.
In many Naruto fanfictions where Naruto meets Kushina and Minato, it often creates a very emotional scenario. Naruto finally gets to meet his parents who he has only heard about. It could be a heart - warming reunion where they share stories, hugs, and Minato and Kushina get to see how far Naruto has come.
In Naruto, the story is built on themes of family, love, and sacrifice. Minato and Kushina are important figures in Naruto's family tree. Minato was a great Hokage and Kushina was a strong - willed woman. Their love for Naruto is a fundamental part of the story's emotional core. So in fanfiction, it's hard to imagine a deviation from this where they would abuse Naruto. Most fanfiction writers prefer to stay true to the spirit of the original story, which is full of positive family values.
One reason could be Naruto's journey. Naruto has to face so many challenges and grow from a lonely outcast to a hero. Minato might be more drawn to this growth and transformation, which is a big part of Naruto's story in fanfics.
They might be quite surprised. Minato could be amused to see different interpretations of his heroic deeds. Kushina might be a bit shocked at some of the wild ideas about their family or Naruto. For example, she might not expect some of the out - of - character scenarios written in the fanfiction.