Perhaps in the fanfiction, Ron's jealousy gets the better of him. He might be jealous of Hermione's success or her friendships with other characters like Harry or some of her academic peers. His jealousy could manifest in ways that make Hermione feel suffocated or unappreciated, causing her to end the relationship. Also, Ron may have some habits that Hermione can't tolerate anymore. For example, his slacking off on certain responsibilities or not taking things as seriously as she does.
One possible reason could be that in the fanfiction, Bill might be depicted as more mature and worldly compared to Ron. Hermione may be attracted to his experience and the different perspective he can offer.
In fanfiction, this kind of plot twist can be really interesting. It might be due to the author wanting to explore different character dynamics. Maybe Hermione realizes she has a stronger connection with Sirius in terms of his more adventurous spirit or his different life experiences compared to Ron.
The relationship changes drastically. Hermione may start to view Ron as someone she once knew well but has now grown apart from. Ron, on the other hand, may go through a range of emotions from denial to anger to sadness. Their friendship that was the foundation of their relationship may also be damaged, and they may have to re - evaluate how they interact with each other in the wizarding community.
One reason could be that Sirius represents a sense of freedom and rebellion that Hermione finds appealing. He's been through a lot and has a certain non - conformist attitude. In contrast, Ron can be a bit more traditional in his views at times.
The relationship between Hermione and Ron in this fanfiction takes a sharp turn. It goes from a relationship (presumably) full of the usual bickering and affection they have in the original series to a complete break - up. Hermione's feelings shift towards Bill, leaving Ron behind.
One reason could be for the sake of creating drama. By showing Ron as controlling, it adds tension to the story. Another might be a misinterpretation of his character traits. If an author emphasizes his protective side too much, it could come across as controlling.
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.
In these fanfictions, a common theme is change. Hermione's life changes drastically when she leaves Ron for Charlie. There may be a theme of new beginnings, as she embarks on a relationship with someone very different. The idea of finding a better fit is also there. Maybe she realizes that Charlie understands her more or can offer her more support in certain areas. Additionally, themes related to family and acceptance can come up, especially if the story delves into how the Weasley family reacts to Hermione's choice and how Charlie's side of the family accepts her.
It could be that Ron is having a bad day or going through some personal issues. And instead of dealing with them in a healthy way, he takes it out on Hermione. For example, if he's feeling insecure about his place in the group, he might lash out at her.
Yes, it can be a somewhat popular trope. Some fans might like the idea of Hermione ending up with Charlie because he represents a different kind of character compared to Ron. Charlie is often seen as more adventurous and perhaps less argumentative. It allows for new storylines and character developments that deviate from the original series. Writers can explore how Hermione's relationship with Charlie would change her and how their combined skills and personalities would interact in different ways than her relationship with Ron did.