Perhaps there were some secrets that Ron was keeping from Harry. In the wizarding world, secrets can be a big deal. If Ron had a secret related to his family or some other important matter and Harry found out about it in a way that made it seem like Ron was hiding something bad, it could lead to Harry not trusting him. Also, Ron's friendship with other characters might make Harry jealous or suspicious. If Ron was getting closer to someone Harry didn't trust, it could spill over and make Harry question Ron's loyalty to him.
It could be that in the fanfiction, there were prophecies or visions involved. Maybe Harry had a vision that made him think Ron would betray him in some way. Or there was a prophecy that Ron was involved in that made it seem like he had an ulterior motive. Additionally, the actions of Ron's siblings could play a role. If one of Ron's siblings got into trouble and Ron's reaction to it made Harry think he was involved in something shady, that could cause the lack of trust.
Another reason could be personal insecurities on Harry's part. Harry has been through a lot and might be over - analyzing Ron's behavior. He could be projecting his own fears onto Ron. For example, if Harry is worried about being betrayed again like he was by some people in the past, he might start to doubt Ron even without solid evidence.
One possible reason could be miscommunication. Maybe Ron said something that Harry misinterpreted, leading to the distrust. For example, if Ron made a joke about Harry's abilities in a time of stress, Harry might take it the wrong way.
It could also be due to a series of unlucky events. Suppose Hermione's well - intentioned plans keep backfiring in a way that makes Harry question her judgment. Like when she tries to solve a problem but ends up making it worse, Harry might start to lose trust.
It could also be due to some actions of McGonagall that seemed suspicious to Harry. Perhaps she made a decision regarding Hogwarts or Harry's safety that Harry didn't fully understand at the time. In the complex world of the wizarding school, decisions are often made with many factors in mind, but Harry might not be aware of all of them, leading to his distrust.
One possible reason could be that Harry discovers some hidden actions of Dumbledore that seem manipulative. For example, if Dumbledore has been keeping secrets about Harry's past or his destiny without fully explaining them. Harry might feel used or deceived, leading to his distrust.
The dynamic becomes very strained. Harry's lack of trust in Ron means that their usual banter and easy camaraderie is gone. Hermione will be stressed because she values both of their friendships. She may end up spending more time with Harry to try to help him through his doubts, which could make Ron jealous. Ron, on the other hand, might become defensive and start acting out, which only makes Harry trust him less. This could lead to a cycle of misunderstandings and a breakdown of their friendship triangle.
It could also be due to a long - standing pattern of behavior. If Ron had been constantly critical or undermining of Harry in the fanfiction, Harry might reach a point where he just can't forgive. Say Ron always made fun of Harry's decisions in front of others, which gradually eroded Harry's trust and patience.
It could be due to a misunderstanding that got out of hand. Ron and Hermione might have been involved in something that Harry misinterpreted as a sign of their disloyalty. For instance, they were secretly working on a project that Harry thought was against him, but in reality, it was for his own protection. However, because of the lack of communication, Harry's anger and unwillingness to forgive grew.
Another reason could be a misunderstanding. Maybe he overhears something Harry and Hermione say and misinterprets it as them not wanting him around anymore. So, in a fit of anger and hurt, he leaves. This can lead to a lot of angst and drama in the fanfiction as the other characters then have to figure out how to get him back.
It could be a misunderstanding. For example, Ron might think Harry did something really bad to Hermione or their other friends. Without proper communication, his anger could lead him to punch Harry. Also, if there was some sort of magical influence or dark magic at play that affected Ron's judgment, that could be a reason too.