Maybe it's because she sees Naruto as her only family left. Having lost so much, she's extremely protective of him. She doesn't want to lose him like she lost others, so she becomes possessive. Also, Naruto has a special power within him, and she might be possessive to keep him safe from those who would try to use him for his power.
In the fanfiction, Kushina's possessiveness may stem from her deep love for Naruto. She had a difficult past, and Naruto is her precious son. She may also be possessive because she knows the hardships Naruto will face in the ninja world. She wants to shield him from all the bad things. Additionally, she might have a fear of Naruto getting influenced by the wrong people or ideologies, so she tries to keep him close and under her influence all the time.
One reason could be that she lost so much time with Naruto due to her early death in the original story. So in the fanfiction, she's overcompensating for that lost time by being possessive.
In the fanfiction, Kushina's possessiveness could be manifested in various ways. She might be very controlling of Naruto's daily activities. For instance, she would decide what he eats, when he sleeps, and how he spends his free time. Also, if there are any female characters showing interest in Naruto, Kushina would be quick to step in and make it clear that Naruto is 'hers' in a sense. This could create some interesting conflicts in the story, as Naruto might feel suffocated by her over - protectiveness at times, while also understanding that it comes from her love for him.
One possible reason could be for Naruto's protection. Maybe she thought by banishing him from a certain place or situation, she could keep him away from a hidden danger that she knew about. For example, there might be a powerful enemy targeting Naruto specifically, and she believed that by making him leave, he would be harder to find.
One possible reason could be that Kushina has a very strong and intimidating personality. In the Naruto universe, she was known to be a tough kunoichi. Naruto, who might be used to more gentle or reserved people around him, could be scared of her boldness.
One possible reason could be some sort of dark influence or curse that makes her lose her maternal instincts. Maybe a powerful enemy targets her and in order to protect Naruto from further danger related to her, she feels forced to abandon him. Another reason might be that she is made to believe false information about Naruto, such as that he will bring great disaster to the village in the future.
One possible reason could be a mission. In many Naruto - related stories, missions often take characters away from their families. Naruto might have received a highly important and dangerous mission that requires him to be away from Kushina for a long time.
It could also be due to some external influence. Perhaps an enemy used some kind of mind - control or manipulation on Kushina. They made her think that Naruto was a threat, so she decided to disown him. In the Naruto universe, there are many powerful villains who could potentially do this kind of thing.
Maybe she's been possessed. In some fanfictions, characters can be taken over by dark forces. So Kushina could be under the control of some evil entity and that's why she hurts Naruto.
One possible reason could be that she later realizes Naruto's true nature and potential. Maybe she sees how strong and kind - hearted he is despite the hardships he faced. She might regret not being there for him during his growing - up years when he needed a mother figure.
One possible reason could be mind control. Maybe an enemy of the village found a way to manipulate Kushina's mind and make her turn against her own baby. Another reason might be a misunderstanding. For example, she could have been tricked into believing that Naruto was a threat to the village in some way.
In the 'naruto rejects kushina fanfiction', it could be due to a sense of self - discovery for Naruto. He may be on a journey to find his true identity and purpose, and he believes that Kushina somehow stands in the way of that. Maybe he has new friends or allies in the fanfiction that he values more and their ideals are contrary to Kushina's. So, to stay true to these new relationships and his own evolving self, he rejects Kushina.