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.
One possible reason could be her involvement in the wizarding world. If her parents are muggles and they find out about the extreme dangers she faces in the magical realm, they might disown her out of fear for her safety. Another reason could be if she starts to show more loyalty to the wizarding community than to her muggle family, making them feel like she no longer belongs with them.
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.
It could be a revenge plot gone too far. Suppose her parents did something in the past that she just found out about, and in her new dark persona, she decides to take the ultimate revenge. Or perhaps she has fallen so deep into the dark side that she no longer values family and sees them as just another obstacle in her path of darkness.
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 excessive stress. Hermione is known for being a perfectionist, and the pressure to perform well on the OWLS might have gotten to her, causing her to make silly mistakes during the exams.
In fanfiction, she could be injured during a battle against dark wizards. Maybe she was caught off - guard while trying to protect her friends.
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.