In this fanfiction scenario, Snape may become protective of Hermione because she might be involved in some important magical research or discovery. Snape, being a knowledgeable wizard, realizes the significance of her work and decides to protect her from those who might want to stop or steal her ideas. Also, Hermione could be in the middle of a conflict between different wizarding factions, and Snape, for his own reasons, decides to side with her and keep her safe.
At first, Hermione might be suspicious of Snape's new - found protectiveness. But as time goes on, she starts to see that he has good intentions. Snape, on the other hand, gradually warms up to Hermione more and more, sharing his knowledge and advice with her.
Perhaps Snape realizes that Hermione is often a target because of her Muggle - born status. In the wizarding world, there's a lot of prejudice against Muggle - borns. Snape, having experienced his own share of hardships and being an outcast in some ways, might empathize with her situation and thus be protective. Also, her friendship with Harry Potter might make Snape more watchful over her, as Harry is always in the center of many dangerous situations and Hermione is closely associated with him.
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.
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.
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 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 could be related to a difficult potion she's trying to brew. Snape is a potions master, and Hermione might be facing an ingredient or a process that she can't figure out on her own. For example, a rare potion that requires precise handling of dangerous ingredients.
Maybe Snape is involved in some sort of mystery or danger within Hogwarts that has to do with magic spells. Hermione, with her vast knowledge of spells and her in - depth study of the Hogwarts library, could be a great help to him in deciphering old spells or magical texts related to the situation.
One possible reason could be that in the fanfiction, the author might explore Snape's complex past with Harry's mother, Lily. Snape's love for Lily could translate into a more nuanced view of Harry. He may see parts of Lily in Harry and that softens his hatred.