It could be due to a prophecy or a magical bond. Maybe there is a prophecy that says something about Hermione being in close physical proximity to Snape at a certain time. Or there is a magical bond that Snape discovers and he thinks that forcing her to sit on his lap is the only way to activate or understand this bond better, although these are all very strange and complex reasons within the realm of fanfiction.
Well, one way it could develop is that Hermione might initially be extremely resistant and angry. She could try to use her magic to get away from Snape. But then, perhaps Snape has some ulterior motive that is not as malicious as it seems at first, like he's trying to protect her from something and sitting on his lap is just a strange way of keeping her close. So, over time, Hermione might start to understand his intentions and they could form an unlikely alliance within the context of the fanfiction.
One possible scenario could be a moment of intense conversation where Hermione is sitting in Snape's lap and they are having a deep discussion about magic theory that only they understand. It might be a turning point in their relationship in the fanfiction world.
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.
A possible plotline could be a moment of comfort. For example, if Hermione is upset or stressed, she sits on a friend's lap (maybe Harry or Ron) for solace and they have a heart - to - heart conversation. Another could be a romantic plotline where she sits on the lap of a love interest as a flirtatious or affectionate gesture, leading to a deeper exploration of their relationship.
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.
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.
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.