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.
Naruto would probably be very confused at first. Kushina is his mother, so her hurting him would go against his understanding of their relationship. He might initially try to talk to her, to find out what's wrong and if there's a way to fix it.
Well, it could have a huge impact. If Kushina hurts Naruto in the fanfiction, it might make Naruto more distrustful. He could become more withdrawn and have to work harder to build relationships later on.
Naruto might initially be in shock. He has always idolized his mother, so if she suddenly hurts him, he would be completely taken aback. He might freeze for a moment, unable to believe what is happening.
Perhaps she's under the influence of a powerful genjutsu. Genjutsu can distort one's perception and make them do things they wouldn't normally do. In this case, Kushina might be seeing Naruto as an enemy in the genjutsu and thus hurt him. Or it could be a test in a very extreme way. She might think that by putting Naruto through pain, she can make him stronger, although this is a very cruel way of thinking.
Another aspect could be that the fanfiction delves into how Kushina's presence affects Naruto's relationships with others. For instance, his relationship with Sasuke could be different. With a strong maternal figure in his life, Naruto might be more stable emotionally and be able to approach Sasuke in a more understanding way, which could potentially change the whole dynamic of their rivalry and friendship.
In some fanfictions, the authors might take creative liberties and create alternate universes where Naruto and Kushina interact in different scenarios. For example, they could be on a joint mission together, and through that, we see their personalities shine. Kushina's fiery nature and Naruto's determination would make for an interesting dynamic.
One possible fanfiction could be about Kushina finding Naruto in a situation where he's lost in a forest. Maybe she uses her unique chakra sensing abilities to track him down. It could be a heartwarming story of a mother reuniting with her son.
In certain fanfic concepts, Naruto could have found a special relic or artifact. This relic might have the power to reverse death. Since Naruto is always on adventures and exploring different places, he might stumble upon such an item and use it to revive Kushina. It could also be related to the Uzumaki clan's secrets that he uncovers which enables him to bring his mother back.
Minato could be a common character. Since he was Kushina's husband in the original story, his presence might add some interesting dynamics to the harem scenario. Another could be Jiraiya. His connection to Naruto and his reputation as a bit of a ladies' man could make for an interesting addition to the story.