Some writers might create such fanfiction for the shock value. They want to break the established strong friendship in the original series to get readers' attention.
It also shows the freedom and diversity in fanfiction. While it might not be in line with the original spirit, it allows for different creative ideas to be explored. However, it can cause some division within the fan community between those who support this new take and those who firmly oppose any deviation from the original relationships.
It can divide the fandom. Some fans who love the Hermione - Ron relationship in the original series might be put off by such fanfiction. They might feel that it goes against the spirit of the story they love.
Well, a frequent theme is the exploration of how their friendship breaks down. It could start with a single event that causes Ron to turn against Hermione. Also, there might be themes related to unrequited love from other characters. For instance, if another character has feelings for Hermione and Ron's hatred is somehow related to that. And sometimes, the theme of betrayal might be there. Maybe Hermione does something that Ron perceives as a huge betrayal, and that's the start of his hatred in the fanfiction.
Some people might create 'Ron hates Hermione' fanfiction because they like to explore alternative storylines. Maybe they want to see how the story would develop if there were more conflict between the two characters. It could also be a way to show different interpretations of their relationship.
One reason could be a divergence from the original character traits. In the original story, Hermione and Ron have a complex relationship with love and friendship. Fanfiction that misrepresents Hermione's feelings towards Ron might go against the essence of her character. Maybe the fanfiction portrays her as hating Ron in a way that doesn't fit with her growth and development in the official series.
Another aspect could be Ron's jealousy issues. In the original story, Ron sometimes gets jealous of Hermione's friendship with Harry or her academic achievements. In fanfiction, this jealousy could be exaggerated to a point where Hermione can't tolerate it anymore and thus is written as hating him. For example, if Ron constantly tries to undermine her success out of jealousy, it would be a valid reason for her to develop strong negative feelings towards him.
One reason could be a divergence from the original friendship dynamic in the books. In the canon, Harry and Ron are good friends, and fanfiction that shows Harry hating Ron might go against what many fans consider the essence of their relationship.
One common plotline could be Hermione forming a closer friendship with Fleur. Since she hates Ron, she might turn to Fleur for support and they could end up having adventures together, perhaps in the wizarding world outside of Hogwarts.
Fanfiction.net could also be a place to search. However, the tagging system there might not be as detailed as AO3. You may need to dig a bit deeper by using the search function and looking through different categories related to Harry Potter fanfiction.