Fear could also be a factor. If the Weasleys fear for their own lives or the lives of their family members due to Harry's association with danger, they may choose to betray him. For instance, if Voldemort is targeting Harry and anyone associated with him, the Weasleys might think that distancing themselves from Harry is the only way to keep their family safe.
One possible reason could be jealousy. If Harry gets more attention or power than the Weasleys, they might feel envious and turn against him.
One reason could be jealousy. In some fanfictions, if Harry has become very famous and powerful, he might think the Weasleys have different motives for being around him, like using his fame for their own gain. So he starts to lose trust in them.
In this fanfiction, perhaps there's some miscommunication. Maybe the Weasleys' boisterous nature is misconstrued by Harry. For example, their large family dynamic might be overwhelming to him at times, which could be a reason for the 'hate' in the fanfic.
One possible reason could be that Harry has post - war trauma. He might have seen so much death and destruction during the war that he doesn't want to bring his darkness and pain to the Weasleys. He may think that staying away is a way to protect them from his own emotional turmoil.
Another reason could be outside influence. If there were dark forces trying to drive a wedge between Harry and the Weasleys, they might plant false ideas in Harry's mind. This could make Harry question the Weasleys' loyalty, even though they are truly his friends.
Maybe in that fanfic, there are misunderstandings between Harry and the Weasleys. For example, they might have had an argument over something like magic use or family values that led Harry to dislike them.
Power could be a reason. If there's a chance for Ron and Hermione to gain a great deal of power by turning against Harry, some fanfictions might explore this. For instance, a new dark power might offer them something they've always desired, like the ability to reform the wizarding world in their own image. They might think Harry would oppose this, so they betray him to get what they want.
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.
One possible plot could be that the Weasleys might betray Hermione over a powerful magical artifact. Maybe they believe she has become too power - hungry in her pursuit of knowledge about it and they side with those who want to keep it away from her.
It could be due to a misunderstanding. Suppose Ron overhears a conversation between Harry and Hermione that he misinterprets completely. He might think they are plotting against him or don't really consider him a true friend. So in a fit of anger and hurt, he decides to betray them.
One possible plotline could be that the Weasleys are secretly working for a dark wizard. They pretend to be friendly with Harry but are actually spying on him for the enemy. Another could be that they have a family secret that would put Harry in danger if he trusts them too much, like a curse in their bloodline that could harm Harry.