Maybe in the fanfiction, Ash is misinformed. Someone tells him false stories about Cynthia planning to stop him from achieving his dreams. Ash, being young and a bit gullible, believes these lies and betrays Cynthia by siding with her enemies. Later, when he discovers the truth, he has to find a way to make amends. It could involve a long and difficult journey of self - discovery and redemption for Ash.
One possible plotline could be that Ash is influenced by a powerful and evil force. This force convinces Ash that Cynthia is somehow a threat to his ultimate goal, like becoming the greatest Pokémon trainer. So, he betrays her by stealing a crucial Pokémon of hers during a big tournament. This would lead to a lot of drama and conflict as Cynthia would be shocked and heartbroken, and Ash would start to realize the error of his ways as the evil force begins to control him more.
In this fanfiction, it could be that Ash is in a situation where he has to choose between Cynthia and his hometown. His loyalty to his hometown is so strong that he feels he has no choice but to betray Cynthia. For example, if Cynthia's actions were unknowingly endangering Ash's hometown in some way, Ash might decide to side against her to protect his home. But this would create a complex web of emotions and relationships that would need to be resolved throughout the story.
It could be a jealous fellow trainer. There are always those in the Pokémon world who envy the skills and achievements of others. This trainer might see Ash and Cynthia as obstacles to their own glory and decide to betray them to get ahead.
Perhaps in the fanfic, Ash betrays Tyson because of a misunderstanding. There could be a false rumor spread about Tyson being involved in some unethical Pokémon training methods. Ash, believing the rumor without fully investigating, turns his back on Tyson. Later, when he realizes the truth, he has to work hard to regain Tyson's trust.
One possible plotline could be that Ash and Cynthia are betrayed by a powerful villain who they once trusted. This villain might have been secretly working against them all along, perhaps stealing their Pokémon or sabotaging their quests. For example, the villain could be a rival trainer who was jealous of their success.
One possible plotline could be that Naruto is somehow transported to the MLP world. He might be tricked into believing the ponies are a threat and thus 'betray' them by taking actions against them. For example, he could be misled by a dark magic user in that world who convinces him the ponies are hoarding power that could be dangerous.
Another plotline might be that Percy betrays God out of a sense of self - preservation. Suppose there's a great threat coming, and Percy believes that siding with an enemy of God is the only way to save himself or the world he cares about. He might think that God's power is not enough to protect them, and so he makes a deal with the other side, which is seen as a betrayal.
One possible plotline could be that Harry betrays Draco in a duel. Maybe there was some sort of misunderstanding or a secret deal that Harry made with someone else that required him to turn on Draco during the duel. For example, if there was a dark wizard threatening Harry's friends and the only way to save them was to sacrifice Draco in the duel by using a forbidden spell against him.
One possible plotline could be that Ash and Cynthia are betrayed by a powerful and long - hidden organization in the Pokemon world. Maybe they are lured into a trap under false pretenses. For example, they are told there is a great Pokemon discovery but it's actually a setup to steal their Pokemon or get some important information from them. Another could be a betrayal from a so - called friend. This friend might be jealous of their achievements and decides to side with an enemy to bring them down. This could lead to Ash and Cynthia losing battles they should have won and facing challenges that seem insurmountable at first.
A plot could involve the Leaf Village betraying Naruto when a powerful and ancient curse is placed on him that makes him seem like a mind - controlled puppet of a great evil. The village, in its panic, decides to take preemptive action against Naruto without fully investigating. This could lead to Naruto having to fight not only the external forces that are trying to use the situation to their advantage but also his own former comrades from the Leaf Village who are now hunting him down.
Perhaps Harry discovers a dark secret about the side he has been fighting for. This discovery makes him feel betrayed and he turns to Tom thinking that Tom offers a different kind of solution. It could be that Tom presents himself as a reformed person or has a convincing argument about a new order in the wizarding world that Harry, in his disillusioned state, starts to believe. Then, the story could follow how Harry's relationships with his old friends deteriorate as he gets closer to Tom's side.
It could also be due to some external influence. Perhaps an evil organization has manipulated Ash in some way, forcing him to act against Alola. They might have used mind control or blackmailed him with something he holds dear, like the safety of his Pokémon.