Another aspect could be Ron's initial immaturity. In the early days, Ron can be quite childish and jealous at times. Hermione's parents, being normal, practical Muggles, might not like these traits in a potential partner for their daughter. They might worry that Ron's immaturity could lead to Hermione being unhappy or held back in some way.
One possible reason could be social class differences. Hermione's parents are Muggles and might view Ron's large, wizarding family with some prejudice. They may see the Weasleys as a bit chaotic and irresponsible because of their financial situation.
Perhaps they misinterpret Ron's intentions towards Hermione. They could think that Ron is not ambitious enough for their highly intelligent daughter. They might expect someone more academically focused like Hermione to be with a more studious partner rather than Ron, who is sometimes seen as more laid - back in his studies.
It's possible that Harry hates it because he has a certain image of Ron and Hermione in his mind. Fanfiction can sometimes create versions of them that are completely different from what he knows. For example, some fanfiction might focus on a love triangle that he doesn't think exists in their real relationship. This divergence from his perception could lead to his dislike.
One possible reason could be a misinterpretation of their relationships. Maybe the author wants to explore a different dynamic where Harry and Hermione bond over Ron's perceived flaws, like his occasional laziness or his insecurities that might come across as annoying to them in this fictional scenario.
Well, in some fanfictions, it could be that they're under the influence of some dark magic or a spell that makes them turn against Hermione. Or perhaps they're misinformed about her actions at Hogwarts and think she's a bad influence. Also, some writers might use it as a plot device to create a rift and then build a story around Hermione trying to fix things with her parents.
Perhaps in this fanfiction, her parents have some secrets that Hermione discovers. These secrets could be related to the wizarding world and their connection to it. Their attempts to hide these things from her make her feel betrayed and thus lead to her hatred. For instance, if they knew about some dark magic in their family history but kept it from her.
Some people might just want to explore alternative scenarios for the sake of creativity. They may think it's interesting to see how the story would unfold if there was animosity between them.
They might hate each other because of differences in their personalities. Austin could be more outgoing and impulsive while Ally is shy and methodical. These differences might lead to clashes. For instance, Austin might make decisions on the fly that Ally doesn't agree with, causing friction. Also, jealousy could play a part. If one of them gets more attention or success than the other, it could breed envy and hatred.
Perhaps in this fanfic, the Weasleys have some actions that conflict with Harry and Hermione's values. For example, if the Weasleys were to make a deal with a dark wizard for the sake of their family's safety and Harry and Hermione found out. This could lead to a rift and hatred. Also, it could be related to a love triangle situation in a very convoluted way. Maybe one of the Weasleys unknowingly gets in the way of Harry and Hermione's relationship, or vice versa, which causes animosity.
It could be that they lied to Harry about something really significant. In the wizarding world, trust is built on truth - telling. If they told a big lie, like about the true nature of a magical object that was putting Harry in danger, he would be very angry. And if their actions led to the death of someone Harry cared about. For instance, if they made a wrong move during a fight that caused an ally to die, Harry would find it very hard to forgive them.
Well, it could be related to Ron's insecurities. He might be insecure about his place in the group with Harry and Hermione. Hermione is always so confident and sure of herself, and Ron might feel that he doesn't measure up. This could translate into aggression as a way of lashing out. Also, perhaps there's some external influence, like a dark magic object that's affecting Ron's mood and behavior towards Hermione.
One reason could be to create drama. The shock value of Ron cheating on Hermione would be high, and it could lead to a lot of emotional turmoil in the story.