One possible reason could be to gain attention or sympathy from a particular character. Maybe she wants to make someone jealous or test their loyalty.
In some fanfiction scenarios, Ginny might fake her pregnancy to manipulate a situation in her favor. It could be related to inheritance, power dynamics within a group, or to secure a relationship. For instance, if there is a love triangle and she wants to keep one person from leaving her, she might think that a fake pregnancy will tie them to her. Also, it could be due to a miscommunication or misunderstanding that spirals out of control, leading her to continue the facade.
Sakura might be waiting for the right moment. In the fanfiction, perhaps she wants to tell him when they are in a more peaceful and stable situation. She could also be concerned about the safety of the baby. If there are threats around, she might not want to make the baby a target by making the news public, even to Sasuke.
Hermione could be hiding her pregnancy from Snape because she's not sure of his feelings towards her. There may be an ambiguous relationship between them. She might be in love with him, but not sure if he feels the same. And she doesn't want to burden him or make things awkward between them. Or perhaps she suspects that Snape might be involved in something dangerous, and she doesn't want her unborn child to be dragged into it.
In some fanfictions, it could be related to a power struggle within the wizarding world. If Hermione was seen as a threat to Ginny's family or her own standing in the magical community, Ginny could be pushed to the edge. Also, if there was a rift in their friendship over ideological differences, like Hermione's more logical and rule - based approach clashing with Ginny's more impulsive and instinctive nature in a very extreme way, it might lead to such a tragic outcome.
One reason could be that the fanfiction is set in a different universe where the characters' personalities are slightly altered. So, this 'new' Harry just doesn't click with Ginny in that version. For instance, if the fanfic is a darker take on the story, Harry might be more jaded and not open to a relationship with Ginny.
In some fanfictions, Ginny may reject Harry because she wants to focus on her own personal growth. She might feel that a relationship with Harry at that time would hold her back from achieving her own goals, like becoming a great Quidditch player or an accomplished witch in her own right. So, she decides to reject him to pursue her own dreams first.
One possible reason could be miscommunication. Maybe Ginny feels neglected by Harry because he's so caught up in his own heroic deeds and doesn't pay enough attention to her. So she might seek comfort or excitement elsewhere.
One possible reason could be a misunderstanding. Maybe Harry was wrongly accused of something, and Ginny's temper got the better of her. For example, if there were false rumors about Harry being unfaithful in the fanfiction world.
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.
Another reason might be some kind of external influence, like a powerful dark magic that clouds Harry's judgment. However, this is often a stretch from the original concept of the characters. Fanfiction writers sometimes like to explore extreme scenarios to add drama to the story.
The idea of subverting the expected relationships is appealing. Instead of the more common pairings, this pairing offers a fresh take. It also allows for a deeper exploration of the characters' personalities. Since they are so different, seeing them come together in a romantic way can be really interesting for fans who want to see new aspects of their favorite characters.