Well, it could be due to the way the Cullens interact with the other people in Forks. If Bella notices that they are being manipulative or using their powers in a way that she doesn't like to influence the townspeople, that could make her start to hate them. Also, if there is some kind of power struggle within the Cullen family that starts to affect Bella negatively, like she gets caught in the middle of it or she sees how it's causing harm to those she cares about, she might turn against them.
It's possible that in the fanfiction, the Cullens are involved in some sort of betrayal. For example, they might side with someone who is against Bella. This act of betrayal would be a strong reason for her to hate them. In fanfiction, new storylines can be created where the Cullens' actions are not in line with what Bella expects or desires, which can result in her hatred.
In some fanfictions, Bella might accidentally harm one of the Cullens, either physically or emotionally. This could lead to them hating her. Also, if she were to side with their enemies in a conflict, that would definitely make them dislike her. For example, if there was a feud between two vampire groups and Bella chose the wrong side, the Cullens would not be happy with her.
In some fanfictions, the Cullens might hate Bella because of jealousy. Maybe other vampires are jealous of the attention she gets from Edward. Another reason could be that she brings too much danger to their family. Her presence could attract unwanted attention from other vampires or vampire hunters, which puts the Cullens at risk.
In the original Twilight story, Bella is initially attracted to the Cullens, especially Edward. But in this fanfiction where she starts to hate them, it's a complete contrast. It shows a different side of Bella's character, one that is more critical and less forgiving. In the original, she is more likely to overlook their flaws because of her love for Edward. Here, she focuses on the negative aspects.
One possible reason could be that she discovers some dark secrets about the Cullens. Maybe she finds out they have been lying to her about their true nature in a more malicious way than in the original story. Another reason might be that she feels threatened by their vampire powers. If they accidentally or purposefully use their powers around her in a menacing way, it could make her feel scared and turn that fear into hatred.
In this fanfiction, it might be that Bella has received some sort of premonition. She could have seen a future where staying with the Cullens leads to their destruction. Out of love for them, she chooses to say goodbye. She might plan to go far away, change her identity, and live a life away from the supernatural world that has been both alluring and dangerous for her.
One possible reason could be that the Cullens make a decision about her future without consulting her. For example, if they decide she should leave Forks for her own safety without asking her how she feels about it.
In the 'the cullens kidnap bella fanfiction', it could be that the Cullens have some hidden motives. They might believe that Bella has a special power or connection that they need to study or keep safe. So they take her against her will, which then leads to a complex relationship dynamic between Bella and the Cullens as she tries to figure out what they really want.
In the 'bella is kidnapped by the cullens fanfiction', one possibility is that Bella is kidnapped to be part of some vampire ritual. However, she refuses to cooperate at first. The Cullens then have to try and convince her that it's not as bad as it seems. This could involve showing her their more human - like qualities, like their love for art and music. As Bella spends more time with them, she may start to see them in a different light and even develop feelings for one of them.
I'm not sure specifically as there could be many different takes on it. Generally, it might be about Bella using her singing in a context related to the Cullen family in a fan - created story.