Some fanfiction writers might do it for shock value. They want to create a very different and extreme situation compared to the original story to attract readers' attention.
Hermione is also very confident in her knowledge. Snape is an expert in Potions, but Hermione is no slouch either. So, when Snape tries to undermine her in Potions class, she might respond with sarcasm. For example, if Snape corrects her in a way that she thinks is wrong, she could say 'Of course, Professor, your infinite wisdom must be right' in a sarcastic tone.
In fanfiction, sometimes authors like to break boundaries and explore 'what - if' scenarios. For the 'Snape cheats on Hermione' idea, it could be a way to create drama and conflict. They might be interested in exploring the emotional turmoil it would cause for Hermione, or how it could change the relationships within the wizarding world they've created in their story. It could also be a way to subvert the typical hero - heroine relationships we see in the original series.
One possible reason could be a misunderstanding. Maybe Hermione thought Snape was involved in some dark plot against her friends. In the wizarding world, there are often secrets and deceptions, and if she got some false information, it could lead her to take such drastic action.
In fanfiction, such a plot is often a deviation from the original spirit of the 'Harry Potter' series. J.K. Rowling's original works do not have such a relationship. Fanfiction sometimes creates extreme or untrue scenarios for the sake of creating drama or exploring different relationships. So, in the context of the official story, it's not true.
One reason could be Snape's knowledge. Hermione, being a knowledge - seeker herself, might feel safe knowing that Snape has a vast understanding of magic, especially the darker aspects. So, she believes he can keep her safe from any threats related to that knowledge.
One possible reason is Snape's prejudice. He might have some sort of bias against Gryffindors in general, and Hermione, being a very Gryffindor - like student, gets the brunt of it. He may expect her to make mistakes typical of Gryffindors like being overly bold without thinking. Another factor could be that Hermione is so intelligent and inquisitive that Snape feels threatened. He likes to be in control of the knowledge in his classroom, and Hermione's constant questions and thirst for knowledge might make him act tough towards her.
One reason could be that Snape believes Hermione has knowledge that is dangerous if it falls into the wrong hands. So, he kidnaps her to keep her safe until he can figure out what to do with that knowledge. Another possibility is that he is being controlled by some dark magic or an outside force which compels him to take her against his will.
One reason could be the re - imagining of Snape's character. In fanfiction, he might be kinder and more understanding towards Hermione than in the original story. So, she feels safe in his presence.
Well, it could be that Draco, in his pure - blood pride and influenced by his family's views, targets Hermione. Maybe he uses some dark magic or just plain bullying. Snape, despite his often cold exterior, might save her because he secretly admires Hermione's intelligence or perhaps he has a deeper reason related to his own past with the Gryffindors or his hidden loyalties. For example, he could see a bit of himself in Hermione's determination to succeed in a world that is often against her.
One possible reason is that some fanfiction writers simply want to break the rules and norms of the original story. They see the established relationships as something to be toyed with, without considering the implications. Also, there might be a small subset of people who have some sort of personal issues or biases that they project onto the characters, leading to the creation of such a negative and untrue scenario in fanfiction.