Another reason might be related to Pikachu's own growth. As Pikachu becomes more powerful and self - aware, it might feel that it needs to go on its own journey without Ash. It could be seeking new challenges and experiences that it believes Ash can't provide, like exploring regions that Ash has no intention of going to or training with Pokémon of a different caliber.
Perhaps Pikachu is lured away by something more appealing. There could be a rare and powerful Pokémon or a special kind of food that distracts Pikachu from protecting Ash. Also, Pikachu might have received some false information. For example, it might have been tricked into thinking that Ash has a better chance of escaping the Spearow without it. In the heat of the moment, Pikachu makes a hasty decision based on this wrong information.
One possible reason could be that Ash senses a dangerous situation where Pikachu's full power might attract unwanted attention or put them in more danger. For example, if there are powerful and unseen enemies nearby, Ash might hold Pikachu back to keep a low profile.
Maybe the creator wants to explore a different side of Pikachu's character. In the official story, Pikachu is loyal and friendly, but in this fanfiction, it could be a way to show a corrupted or influenced Pikachu. Also, it could be a way to deconstruct the typical hero - sidekick dynamic between Ash and Pikachu and make readers think about their relationship in a new way.
Perhaps Kakashi feels that Naruto is becoming too reliant on him and not developing his own skills independently. So, in order to force Naruto to become stronger on his own, he decides to abandon him. This is a tough - love approach. Naruto has great potential, but he needs to learn to stand on his own two feet, and Kakashi may think this is the best way to make that happen.
One possible reason could be that Sakura realizes she has different goals or values that are not compatible with being around Naruto. Maybe she discovers a new path in life that doesn't involve him, such as focusing on her own medical ninja career without Naruto's chaotic lifestyle interfering.
It could also be due to external pressure. The media or some powerful figures in the fanfiction world might force Tony to stay away from Peter. They could spread false rumors about their relationship or use their influence to separate them for their own gain.
Ash would be terrified. He is facing a group of Spearow without his most reliable Pokémon by his side. He would also feel disappointed in Pikachu. After all the adventures they've had together, he never expected Pikachu to leave him in such a dangerous situation. He might start to doubt their friendship and partnership.
In some fanfictions, Naruto might abandon his family due to a false prophecy that convinces him he's a danger to them. He may think that by leaving, he can protect them from harm that might come his way because of his past or enemies.
Well, the story could develop in many ways. Ash might go into a deep depression at first. He has been with Pikachu for so long that its absence would be a huge blow. He could start doubting his abilities as a Pokémon trainer.
Well, in this fanfiction, if Ash turns into Pikachu, it might lead to a story where Pikachu - Ash discovers new aspects of the Pokémon world. For example, he could experience the world from a Pokémon's perspective, understanding their fears and joys more deeply. He might also have to deal with other Pokémon who don't immediately recognize him as Ash and react in different ways to this strange Pikachu.