There could be some underlying mental health issues within the family. Perhaps her parents have some undiagnosed conditions that make them more prone to lashing out. In the fanfiction world, this could be explored further, like how Hermione tries to understand and deal with this aspect of her family situation while also dealing with the abuse.
Well, in some fanfictions, it could be that they're under the influence of some dark magic or a spell that makes them turn against Hermione. Or perhaps they're misinformed about her actions at Hogwarts and think she's a bad influence. Also, some writers might use it as a plot device to create a rift and then build a story around Hermione trying to fix things with her parents.
Perhaps in this fanfiction, her parents have some secrets that Hermione discovers. These secrets could be related to the wizarding world and their connection to it. Their attempts to hide these things from her make her feel betrayed and thus lead to her hatred. For instance, if they knew about some dark magic in their family history but kept it from her.
In the fanfiction, it could be related to family values. Draco comes from a family with certain beliefs about blood purity. Hermione being a Muggle - born might be seen as beneath him by his family's standards. So, Draco might abuse Hermione as a way to show his superiority based on those wrong family - instilled ideas.
Well, it could be possible in some really out - there fanfictions. Maybe some writers want to create a lot of drama. They might think that if Hermione's parents suddenly turn against her, it can add some really intense emotional turmoil to the story. But in the original books, Hermione's parents are really supportive.
One possible plotline could be that Hermione is under the influence of a dark spell or curse that makes her lose control and harm her parents. Another could be that she is forced to do it by some dark wizard who has her parents hostage and threatens her with something worse if she doesn't. Or perhaps in a very dark and alternate universe where she has turned to the dark side for some reason and sees her parents as an obstacle to her newfound power.
One possible reason could be social class differences. Hermione's parents are Muggles and might view Ron's large, wizarding family with some prejudice. They may see the Weasleys as a bit chaotic and irresponsible because of their financial situation.
Perhaps some writers are trying to explore extreme scenarios to show the character's resilience. But this is a very wrong approach as it involves abuse. It could also be that the fanfiction community has not been strict enough in policing such content, leading to some writers thinking it's okay to write such things. In a proper fanfiction environment, characters should be respected and not subjected to such inappropriate abuse.
Draco's father, Lucius, might be very disdainful at first. He has a strong sense of pure - blood superiority and Hermione is a Muggle - born. His mother, Narcissa, might be more reserved in her dislike. She could be more focused on how Hermione's presence affects Draco.
One possible reason could be that she is facing a powerful dark wizard who wants to assert dominance over her. Maybe the villain is trying to break her spirit as she is known for her strong will in the original story.
One possible reason could be that in the fanfiction, her parents might have made a sacrifice that left her alone, and she resents them for not being there for her. Maybe she feels abandoned and that has led to her hatred.