Perhaps in the fanfiction, Sirius gets involved with the wrong crowd. And Harry, who has a strong sense of right and wrong, can't stand being associated with Sirius any longer. Or maybe Sirius has a falling out with one of Harry's other important people, like Hermione or Ron, and Harry takes their side and starts to hate Sirius as a result.
It could be that Percy gets involved in a conflict where he sides against Artemis' hunters. His actions in this conflict might make Artemis view him as an enemy. For example, if he accidentally harms one of her hunters while trying to protect someone else, she may grow to hate him.
Perhaps in the fanfiction, Scott falls in love with someone that Stiles also has feelings for. This love triangle situation could turn Stiles' feelings into hatred as he sees Scott as a rival for the person he cares about. Also, if Scott accidentally reveals one of Stiles' secrets that he promised to keep, Stiles would likely be very angry and could start to hate Scott.
One possible scenario is that Minato was resurrected in a wrong way in the fanfiction. His behavior after resurrection was not what Naruto expected. Maybe he was more cold - hearted or seemed to have different priorities that didn't include Naruto, so Naruto started to hate him.
One possible scenario could be that Sirius Black spanks Harry as a form of discipline for some dangerous or disobedient behavior that Harry has engaged in within the context of the fanfiction's plot.
One scenario could be that Edward accidentally endangers Bella's family in the fanfiction. Since Bella is very protective of her family, this could be a major turning point for her feelings towards him. For instance, if his vampire nature attracts some dangerous vampires to Forks and they target her family.
One possible scenario is if Naruto is constantly blamed for the team's failures. For example, if every mission that goes wrong is pinned on Naruto, even when it's not his fault, he might start to hate the team. This could be due to a misperception by the other members or a malicious plot within the fanfiction world.
One possible scenario is that Jacob reveals a secret that Bella doesn't like. This could make her feel betrayed and start to hate him.
It might also be a situation where Sirius is frustrated with Harry for doing something reckless. He pins Harry down to get him to listen and understand the seriousness of his actions. For example, if Harry was planning to go after a powerful enemy alone without a proper plan.
One possible scenario is when the imprint causes Jacob to be very protective of the person he imprints on. Bella might see this as Jacob neglecting their previous friendship or bond. Another scenario could be that the person Jacob imprints on has a negative attitude towards Bella, and Jacob seems to side with the imprinted one, making Bella hate the whole situation.
One possible scenario could be Sirius noticing how Harry blushes whenever Ginny's name is mentioned. So he might say something like, 'Harry, I've seen you go redder than a Weasley's hair at the mention of Ginny.' Another could be if Ginny gives Harry a special gift, and Sirius teases him about it, 'Harry, that little present from Ginny must mean something special, eh?' A third scenario could be when Ginny is around and Harry acts a bit clumsy. Sirius would then joke, 'Harry, you're more out of sorts than a drunk Hippogriff when Ginny's near.'