In fanfics, the relationships between characters can be explored in new ways. If the fanfic writer decides to show a rift between Hermione and Harry for the sake of drama, they might create situations where Hermione doesn't believe Harry. This could be due to a series of miscommunications or actions that Harry takes which seem out of character to Hermione.
One possible reason is that the fanfic writer wants to create conflict. By making Hermione not believe Harry, it adds tension to the story.
One possible reason could be that Hermione has a different romantic vision. In the original series, she might see Harry more as a friend. In the fanfic, her character could be written as being focused on her own personal growth and goals, which don't align with a romantic relationship with Harry.
One possible reason could be a miscommunication. Maybe Hermione accidentally reveals a secret of Harry's that he didn't want known, and this causes him to feel betrayed and start to hate her. Another reason could be jealousy. If Hermione gets more attention or praise for her magical abilities than Harry in the fanfic, he might develop feelings of envy and hatred. Or perhaps there's a love triangle situation where Hermione is involved with someone Harry doesn't like, and that causes his hatred.
In the fanfic, it might be due to some sort of magic gone wrong. There could be a spell that made her act out of character. It's not unheard of in the wizarding world for spells to have unexpected effects on a person's behavior and relationships.
In 'harry gets rejected by hermione fanfic', perhaps Hermione has a pre - existing relationship or crush on someone else. It could be another character from the wizarding world. Also, she might be afraid of the potential consequences of being in a relationship with Harry, like how it could affect their friendship or the dynamics within their group of friends. She may also be dealing with personal insecurities that make her hesitant to enter into a relationship with Harry.
One possible reason could be that Hermione sees Harry more as a friend. She has a deep, platonic bond with him and doesn't want to risk ruining that friendship by turning it into a romantic relationship. In the original story, they have been through so much together as comrades in the fight against Voldemort, and she may value that camaraderie above all else.
In some fanfictions, there might be a new magical element or prophecy that makes Harry question Hermione's motives. If the prophecy says something about Hermione being involved in a dark plot, even if it's false, Harry may start to have doubts about her.
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 they lied to Harry about something really significant. In the wizarding world, trust is built on truth - telling. If they told a big lie, like about the true nature of a magical object that was putting Harry in danger, he would be very angry. And if their actions led to the death of someone Harry cared about. For instance, if they made a wrong move during a fight that caused an ally to die, Harry would find it very hard to forgive them.
One reason could be her thirst for knowledge. Hermione is known for her love of learning, so she might have decided to learn Bulgarian just for the sake of it. Another reason could be related to a relationship, like with Viktor Krum, which gives her an incentive to learn his native language.