Perhaps Ron was influenced by outside forces. There could have been dark magic or some other characters manipulating him. It's not uncommon in the wizarding world. Some powerful dark wizards might have preyed on Ron's insecurities to make him turn against Harry.
On the other hand, it could also be a turning point for Ron. It might make him determined to make amends. The slap could be the jolt he needs to fight against whatever was causing him to betray Harry in the first place, whether it was jealousy or outside influence.
Jealousy is a big one. Ron often feels like he's in Harry and Hermione's shadow. He might see their achievements and feel left out or unimportant, which could lead to his betrayal.
One way could be through a misunderstanding. Maybe they believe Hermione has done something wrong, like they misinterpret her actions regarding a dangerous spell she was researching. So they distance themselves from her, which she sees as a betrayal.
Well, if Ron and Hermione betrayed Harry in fanfiction, it would completely shatter the core of their friendship. In the original story, their loyalty was unwavering. This betrayal could lead to Harry being isolated and more vulnerable to Voldemort. Maybe he'd be captured more easily without their support. It would also make the readers feel a sense of shock and disappointment as we've come to expect their strong bond.
In a fanfiction with Ron's betrayal, there could be a situation where the magical world turns against Ron. After all, Harry is seen as a hero and Hermione is highly respected. When Ron's betrayal is revealed, the wizarding community might shun him. This could lead to a redemption arc for Ron where he has to prove himself worthy of forgiveness not only to Harry and Hermione but also to the entire wizarding world. He might have to go through great trials and make huge sacrifices to make up for his actions.
In some fanfictions, it might be because of a love triangle gone wrong. Suppose Ron had unrequited feelings for Hermione and thought she favored Harry too much. And Harry, perhaps misinterpreting Hermione's actions in a crucial situation, joined Ron in betraying her. This would be a complex and emotional reason for the betrayal.
One popular one might be 'The Moment of Impact' where the slap is a turning point in their relationship. It delves into the emotions and misunderstandings that led to that moment.
Maybe Ron is under the influence of some dark magic that makes him lose control. It could also be a misunderstanding. Perhaps he thinks Harry has done something really bad to Hermione or their other friends while being pregnant, which enrages him without knowing the truth.
Sometimes it's to show a conflict in their relationship. Ron and Hermione have a complex relationship, and a slap can symbolize a breaking point in their interactions.
There might be some kind of misunderstanding. For example, Ron could have been misinformed about Hermione's actions. He thought she betrayed him in some way, like sharing Gryffindor's secrets with Slytherins, so he lashes out at her.