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 plotline could be that some of the former Death Eaters who were pardoned as part of the post - war reconciliation secretly plot against Harry. They might spread false rumors about him misusing his power or being a threat to the wizarding world again. Another could be that some of his own friends turn on him out of jealousy. Maybe they feel overshadowed by Harry's continued fame and hero status and start to believe the lies being spread about him.
It could be Ron. Maybe he starts to feel that Harry has changed too much after the war and is no longer the friend he once knew. He might be influenced by outside forces or his own insecurities. Another possibility is Hermione. If she gets too involved in the new Ministry politics and starts to believe that Harry's actions are not in line with the new wizarding order, she could turn on him.
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.
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.
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.
It could be some of the pure - blood supremacists who were biding their time. Even though the war was over, they might still resent Harry for his role in defeating Voldemort and decide to get revenge by making it look like he betrayed them. Another possibility is some of the Ministry officials who were corrupt and didn't want Harry's influence around.
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.