In some fanfictions, Ginny might be mad at Harry because of a misunderstanding. For example, she could misinterpret his actions towards another girl as being unfaithful. Maybe he spends a lot of time with Hermione on a project and Ginny thinks there's something more between them.
It could be that Harry has made a decision without consulting Ginny. Like if he decides to take on a very dangerous mission without considering how it would affect her. Ginny, being strong - willed, would be angry that he didn't value her opinion or take her feelings into account.
One possible reason could be that Harry was away a lot after the war, dealing with the aftermath and his newfound responsibilities. Ginny might have felt neglected. Maybe she expected more time together, but Harry was constantly occupied with things like helping to rebuild Hogwarts or being involved in the Ministry's new initiatives.
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 be hurt at first. Since she's pregnant, her emotions could be more fragile, and Harry's anger could really sting.
It might be that there was some sort of misunderstanding. For example, a rumor spread that Harry was getting too close to one of his female colleagues at work. Ginny, being a fiery character, got mad without really knowing the truth. In fanfic, these kinds of misunderstandings are often used to create drama.
One possible reason could be that he's worried about Ginny's safety during the pregnancy. He might be overprotective and his concern comes out as anger.
A third scenario could be jealousy. If other witches start showing interest in Harry and he doesn't immediately and firmly shut it down, Ginny might get mad. She is a proud witch and doesn't want to share Harry's affections, and his seeming indecisiveness in such situations could lead to her anger in fanfiction.
Well, perhaps Ginny is obsessed with Harry fanfiction because she has a deep connection to Harry in the original story. Fanfiction allows her to explore different scenarios and relationships that might not have been fully explored in the official books. It could also be that she simply loves the idea of Harry so much that she wants to read all the different interpretations of him in fan - created stories.
Maybe he loves the idea of the Firebolt in the fanfiction. It could be that the stories expand on his experiences with the Firebolt in a really exciting way.
Perhaps in the fanfiction, Sirius is mad at Harry because Harry has befriended someone Sirius doesn't trust. Sirius, being protective of Harry, might see this new friendship as a threat. He could be worried that this person will lead Harry astray or put him in danger, and his anger towards Harry stems from his concern for Harry's well - being.
It could be a plot device to create drama. For example, if there's a love triangle situation and Harry faints on Ginny, it can stir up all sorts of emotions among the characters involved. It might also be because of some sort of dark magic that has been affecting Harry secretly and Ginny is the trigger point for him to finally succumb to it. This can then lead to a whole mystery - solving plotline in the fanfiction.