One possible reason could be a misunderstanding. Maybe there was a miscommunication about something important, like a plan to fight against the Dark Arts. For example, if the Weasleys accidentally left Hermione out of a crucial part of the plan, she might be very angry.
It could also be because of a difference in values. If the Weasleys have a more laid - back approach to something like schoolwork or following rules, and Hermione, being a stickler for rules and academic excellence, might get mad at them for their lackadaisical attitude.
Perhaps Ron Weasley said or did something really hurtful to Hermione, and she extends her anger to the whole Weasley family because they are so close - knit. Like if Ron made a comment about her Muggle - born status in a negative way, Hermione would be justifiably mad at him and might be mad at the rest of the Weasleys for not correcting him right away.
One possible reason could be jealousy. Maybe one of the Weasleys develops romantic feelings for Hermione and the others don't approve or also have feelings for her. Another reason might be related to her magical abilities. If Hermione has some unique magic or knowledge that the Weasleys think can benefit them or their family in different ways, they could fight over it. For example, her knowledge of ancient spells could be seen as a great asset.
One possible reason could be jealousy. Maybe she sees the Weasleys as getting things too easily in the wizarding world compared to her own hard - earned achievements. For example, they have a big family support system that she lacks, and she might envy that.
Perhaps in this fanfic, the Weasleys have some actions that conflict with Harry and Hermione's values. For example, if the Weasleys were to make a deal with a dark wizard for the sake of their family's safety and Harry and Hermione found out. This could lead to a rift and hatred. Also, it could be related to a love triangle situation in a very convoluted way. Maybe one of the Weasleys unknowingly gets in the way of Harry and Hermione's relationship, or vice versa, which causes animosity.
It could be that in the fanfic's plot, the Weasleys have some actions that go against Hermione's principles. Say, they might be involved in some pranks that have negative consequences for Hermione's studies or personal goals, and that's why she hates them.
One possible plot could be that the Weasleys might betray Hermione over a powerful magical artifact. Maybe they believe she has become too power - hungry in her pursuit of knowledge about it and they side with those who want to keep it away from her.
Well, they could start with an honest conversation. The Weasleys could sit down with Hermione and ask her what's wrong. For example, Ginny could take the lead and say, 'Hermione, we know you're mad at us. Let's talk about it.'
Well, in many such fanfictions, the Weasleys might protect Hermione by using their magic skills. For example, they could create protective spells around her when she's in danger. They might also physically stand in her way when she's facing threats, like some dark wizards. And they would use their family influence in the wizarding world to keep her safe from any kind of harm or discrimination.
Maybe Snape made a snide comment about Hermione's intelligence or her Muggle - born status. Snape has a rather acerbic tongue, and if he said something derogatory about Hermione's abilities or her heritage, she would surely be furious. After all, Hermione works extremely hard to prove herself in the wizarding world and any attack on her intelligence or background would be a big blow.
One possible reason could be a disagreement over how Molly treats Harry. Maybe Molly is being overprotective of Harry in a way that Hermione thinks is hindering Harry's growth as a wizard. For example, if Molly tries to stop Harry from taking certain risks that Hermione believes are necessary for his development.
One possible reason could be a powerful spell gone wrong. Maybe she was caught in the crossfire of a dark magic spell that Snape was either trying to counter or was involved in accidentally. Another reason might be extreme emotional shock. For instance, if she discovered some dark secret about Snape that was so overwhelming it sent her into a catatonic state.