There are several possible reasons. It could be due to a misunderstanding about a prophecy or a magical object. For instance, if Ron accidentally breaks a magical heirloom that Harry believes could be important in fighting Voldemort, Harry might be so angry that he ignores Ron. Also, it could be related to Ron's family. If Ron's family does something that Harry disapproves of, like siding with the Ministry when Harry thinks they're wrong, Harry could start ignoring Ron.
It could be that Harry has been influenced by some external force. In fanfiction, there could be a new dark wizard or a magical object that is affecting his mind. This new influence might make him think that Ron and Hermione are in danger if he stays close to them, so he ignores them to keep them safe.
One possible plot could be that Ron and Hermione start to focus more on their relationship and their own personal growth, which leads them to unintentionally ignore Harry. They might get caught up in their studies at Hogwarts or in exploring their magical abilities further. For example, Ron could be working hard on becoming a better wizard in the areas he's not so confident in, like spell - casting accuracy. And Hermione might be delving deeper into ancient magic research. This causes Harry to feel a bit left out as he's so used to their trio dynamic.
One popular 'Harry is ignoring Ron' fanfiction could be 'The Broken Bond' where the rift between them is caused by a misunderstanding during a dangerous mission. Another might be 'Silent Treatment' which delves deep into how Hermione tries to bridge the gap between them. There's also 'Ignored Friendship' which shows the emotional turmoil Ron goes through when Harry ignores him.
It might be because of a misunderstanding. In some fanfics, a false rumor could spread about Harry. Ron and Hermione, believing it to be true, might decide to distance themselves from him. Say, there's a rumor that Harry is involved with dark magic in some way, and they, being the 'good guys', think they need to stay away.
In fanfiction, Hermione might choose Harry over Ron because Harry often has a more heroic image. He is the Boy Who Lived, and his adventures are often more central to the main plot. Hermione could be attracted to his leadership qualities and the fact that he always seems to be at the heart of the action.
However, from a fandom perspective, it could mean that he missed out on some creative interpretations of his character. Fanfiction often gives new and interesting takes on characters, and by ignoring it, he didn't get to see how fans saw him in different scenarios.
There could be a 'Harry punches Ron' fanfic to explore different character interpretations. Maybe the writer wants to show a more impulsive and hot - headed Harry, or a Ron who has done something really wrong that would push Harry to such an extreme. It can also be a way to break away from the typical, harmonious view of their relationship and see how they would deal with such a violent rupture in their friendship.
However, this could also lead to a strengthening of their relationship between Ron and Hermione. They would have to rely more on each other without Harry around. And when the truth behind Harry's ignoring them is finally revealed, it could either make their friendship with Harry stronger than ever as they work through the issues, or it could be so damaged that it's hard to repair.
Maybe Ron is jealous of Harry and wants to have something that belongs to him for a change. The cloak gives Harry a certain freedom and mystery, and Ron might be longing for that kind of experience.
Because their friendship is so deep - rooted. They've been through so much together, like facing Voldemort. So, even if Ron makes a mistake, Harry knows the overall value of their bond.