One possible reason could be a misunderstanding. Maybe Ginny is under the influence of a powerful dark magic that makes her act against Harry without her full control. For example, a powerful mind - controlling spell could be involved.
Another reason might be a conflict of interests. Suppose there is a situation where Ginny believes that by betraying Harry, she can save her family or the entire wizarding world. It could be a difficult decision for her where she thinks the end justifies the means.
It could be due to jealousy. Perhaps Ginny has been spending a lot of time with another character, and Harry misinterprets their relationship, leading to his anger. In fanfiction, the writers often play with such emotions to create drama.
She might hate it because it could change the way people see her character. In the original story, she is a strong and independent person, but fanfiction might portray her in a wrong way.
One possible plotline could be that Harry is so focused on his fight against Voldemort that he doesn't have the emotional space to develop feelings for Ginny. He might see her as just a friend or even as a younger sister figure. For example, he could be constantly training, researching dark magic defenses, and leading the Dumbledore's Army, leaving no room for romantic thoughts about Ginny. Another aspect could be that Harry has unresolved trauma from his past, like the death of his parents, which makes him hesitant to get involved in a romantic relationship with Ginny.
In some fanfictions, Ginny might be written to betray Harry because of a prophecy. The prophecy could mislead her into thinking that Harry was going to bring doom, and so she decides to act against him to prevent that. It's a different way of looking at their relationship compared to the original story, where their bond was based on love and trust.
Maybe in fanfiction, Ginny discovers that Harry still has some unresolved feelings for Hermione. This could create a rift between them and lead to a divorce. It's not uncommon in fan - created stories to explore complex love triangles that were not fully resolved in the original series.
In some fanfics, it might be for the sake of a more complex plot. The writer wants to create drama and conflict. So, Ginny could be made to betray Harry to add a new layer of struggle to Harry's life. It could also be related to her family loyalty. If the Weasley family was somehow forced into a difficult situation where they had to choose between Harry and their own safety, Ginny might be written to make a choice that seems like a betrayal to Harry.
One possible reason could be that Molly might blame Harry for putting her family in danger. Since he is often at the center of Voldemort - related troubles, her family gets involved in protecting him. For example, during the Battle of Hogwarts, her family was right in the middle of the fight mainly because of Harry. Another reason could be that she blames Harry for Ron's decisions at times. Ron follows Harry on his adventures and sometimes it puts Ron in harm's way, which Molly as a mother may not like.
They could have been disowned if they took a stance on a political or magical issue that was against their parents' beliefs. For instance, if there was a conflict between pure - blood and half - blood wizards, and Harry and Ginny sided with the half - bloods when their pure - blood parents were firmly against it. This difference in ideology could cause a rift so large that their parents would disown them.
Well, one reason could be jealousy. Maybe his friends envy his fame or special abilities. Another could be misinformation. If they are fed false information about Harry, they might turn against him. For example, if someone convinces them that Harry is secretly working for the enemy.
One reason could be that the fanfiction changes her personality traits. In the original, she's kind - hearted but in fanfic, she might be made mean. Harry, who knows the real Molly, won't like this new version.