There's a chance that some of his former teammates who have their own insecurities. For example, someone who always felt in Naruto's shadow might be the one to betray him in order to get more recognition for themselves.
In such a fanfiction, one possible plot could be that the villages Naruto protected during the war start to fear his power. They might see him as a threat now that the common enemy is gone. So, they conspire against him, leading to his feeling of betrayal.
Another reason could be political. After the war, the power dynamics in the village might have changed. There could be new leaders or groups who saw Naruto as a threat to their authority. They might have plotted to make him look bad and turn the villagers against him, causing him to feel betrayed by the very people he fought for.
Naruto might initially be in shock. He has always believed in his friends, so the betrayal would be a huge blow. He could try to reason with them at first.
He would probably be heartbroken at first. Naruto has always been someone who values his bonds with others. Being betrayed by the village he loves would be a huge blow. He might isolate himself for a while, trying to figure out what went wrong.
Well, in some fanfictions, it could be due to political manipulation. After the war, there might be different factions vying for power. Naruto could be seen as a threat to certain groups, and they manipulate his friends into betraying him. Also, sometimes characters' insecurities can play a role. His friends might have their own personal issues and insecurities that lead them to turn on Naruto.
One possibility could be that the wizarding world turns on Harry after the war. Maybe they start to fear his power and influence. They could spread false rumors about him being a dictator in the making. His friends might be pressured to turn away from him, and he would be left alone, feeling betrayed by those he fought for.
It's popular because it adds a new layer of drama. The war is over, but the problems don't end. It shows that even in victory, there can be backstabbing. People like to explore the darker side of human nature in the context of the Potter universe.
There's a fanfic called 'False Alliances'. After the war, Potter discovers that some of the people he worked with during the war had ulterior motives. They betray him to gain control over certain dark magic artifacts that Potter was protecting. It shows how the after - effects of war can create a climate of mistrust and betrayal.
One possible plot could be that the village, still shaken by Pain's attack, blames Naruto for not preventing all the destruction. They think he has some hidden connection to Pain and turn on him. Naruto, heartbroken, decides to leave the village and find out the truth about his past and the Akatsuki on his own.