It could be that a member of the Order, out of jealousy or a wrong perception of Hermione's power, convinces the others that she is a liability. So, they betray her by not believing in her when she discovers something important or by preventing her from using her skills to help. Hermione, being the strong - willed character she is, would then strive to prove them wrong and expose the real issues within the Order.
In fanfiction scenarios where Hermione is betrayed by the Order, it could be related to a prophecy. Maybe there is a prophecy that involves Hermione in a way that the Order misinterprets. They think that by betraying her, they are saving the wizarding world. Or perhaps there is a conflict of ideology. Hermione might have some new ideas or approaches to magic that the more traditional members of the Order oppose. And they see her as a danger to their established ways. Also, personal grudges among the Order members could spill over. If someone in the Order has a long - standing grudge against Hermione for some past incident, they could convince others to betray her.
I'm not sure specifically as there could be many different interpretations in fanfiction. It might be about a situation where The Order, perhaps in the Harry Potter universe, betrays Hermione in some way, like leaving her in a dangerous situation, or not believing her when she has crucial information.
One possible plot could be that Hermione's friends start believing false rumors about her. For example, they might think she's been stealing potions ingredients for personal gain when in fact she was framed. As a result, they shun her and exclude her from their group activities. This makes Hermione feel isolated and betrayed, and she has to find a way to clear her name and either regain her friends' trust or find new allies.
Obviously Hermione is one of the main characters. As for the others, it could include members of The Order such as Dumbledore, Sirius Black if he was still alive in the context of the story, and other Order members who are involved in the act of betrayal.
One way it could develop is if Dumbledore withholds important information from Hermione about her family or her own magical abilities. He might do this because he fears she will use that knowledge in a way that could disrupt his grand plan for the wizarding world. For instance, Hermione discovers she has a special power that could defeat Voldemort easily, but Dumbledore has kept it from her all along, making her feel extremely betrayed as she could have saved many lives if she knew earlier.
She might get revenge because she felt the Order made a decision that put her or her friends in danger. For example, if they sent Harry on a dangerous mission without proper support, Hermione could plan some elaborate scheme to teach them a lesson. Maybe she'd use her intelligence to expose some of their secrets or weaknesses.
One possibility could be that Hermione feels extremely betrayed and isolated. She might then turn to her own studies even more intensively, using magic to create a safe space for herself. Maybe she discovers some long - forgotten magic that could help her in this difficult situation.
In such a fanfiction, perhaps Hermione could be blackmailed into betraying the Order. Maybe an enemy discovers a deep secret about her family and forces her to give up Order secrets.
She would probably be very hurt and angry. Hermione is a strong - willed character, so she might immediately start trying to find out the truth and clear her name.
It could be a new character introduced in the fanfiction. Maybe someone who has infiltrated the Order and wants to create chaos by turning them against Hermione. This person could be a spy from the Death Eaters or another dark - aligned group, with the goal of weakening the Order from within by making them lose one of their most intelligent and powerful members.