Well, it could be many reasons. Maybe there was a misunderstanding. Some dark magic might have been used to make his friends think he did something bad. Or perhaps a powerful enemy framed him by planting false evidence. It's hard to say exactly without more context from the fanfiction.
In this kind of fanfiction, often it's because someone wants to get rid of Harry. It could be a jealous rival within Hogwarts. They might spread lies about Harry being involved in dark arts or some illegal activities against the wizarding world. And with enough false witnesses and planted evidence, he gets betrayed and sent to Azkaban.
It might be due to a prophecy or a power struggle. There could be a group that fears Harry's growing power. They think by betraying him and sending him to Azkaban, they can control the situation. Maybe they convinced others that Harry was a threat, like he was going to turn to the dark side. So, his so - called friends turned against him under this false pretense.
Harry might survive in Azkaban because of his connection to his friends, even if they betrayed him. Their memories could give him the strength to hold on. And he may be able to use his magic in unique ways. He could draw on the positive emotions he has left, like his love for his parents, to create a shield against the Dementors. Plus, he may be constantly looking for a way to prove his innocence and that hope could keep him alive.
One possible plot could be that Harry is framed for a serious crime he didn't commit. Maybe someone has used Polyjuice Potion to look like him while doing dark magic. The wizarding community, believing the false evidence, sends him to Azkaban. In Azkaban, he has to endure the Dementors and try to prove his innocence from within the prison walls.
It could be a jealous classmate from Hogwarts. Someone who has always been in Harry's shadow and finally sees an opportunity to get rid of him. This person might be a Slytherin who has connections with some dark wizards outside of school and they work together to frame Harry.
Another possibility is Dumbledore. In some fan - created stories, if Dumbledore was found to be hiding crucial information about Harry's past or destiny for his own purposes, Harry would surely feel betrayed. After all, Harry had put so much trust in Dumbledore as a mentor.
Another possible betrayer could be Dumbledore in some darker fanfictions. He might have a hidden agenda all along. He could be using Harry as a pawn in a much larger and more sinister game. And when Harry starts to figure things out and goes against Dumbledore's plans, Dumbledore betrays him to keep his own plans on track.
It might be popular because it allows fans to take control of the story. In the official Harry Potter stories, there are set relationships and storylines. But in fanfiction, especially this type, fans can create their own scenarios where they can make the characters face unexpected and difficult situations like betrayal within a harem - like setting. This gives a sense of novelty and freedom to the fans who want to explore the Harry Potter world in a different way.
One possible event could be a false accusation within the wizarding community. Maybe someone frames Harry for a serious crime, and his so - called friends believe it without really giving him a chance to defend himself. Another event could be a power struggle within Hogwarts or the wizarding world in general, where people see Harry as a threat and turn against him to gain an advantage.
Harry may also get angry. He could lash out at the person who he believes has betrayed him. For instance, if it was Ron, he might shout at him and say things he doesn't really mean in the heat of the moment. This anger would be a result of his deep - seated hurt from the betrayal.
Well, in 'harry betrayed by the light fanfiction', it could be that the so - called good guys, who are associated with the light, have their own agendas. Maybe they sacrifice Harry's well - being or safety for the greater good as they perceive it. For example, they might make decisions without his consent that put him in danger, making Harry feel betrayed. It could also be about secrets being kept from him that, when revealed, make him feel like he can't trust those he thought were on his side.
Another theme is revenge. In some fanfictions, Harry seeks revenge on those who betrayed him. It shows his darker side and how far he's willing to go.