One possible reason could be a false accusation. Maybe someone spread a rumor that Naruto was involved in a heinous act, and without proper investigation, everyone turned against him.
One reason could be his power. His connection to the Nine - Tails makes him extremely powerful. In fanfiction, some in Konoha might fear this power and thus hate him. Another reason could be jealousy. Naruto becomes very popular and respected in the original story. But in fanfiction, some characters might be jealous of his achievements and turn that into hatred.
In some fanfictions, Naruto might be hated by his friends due to a misunderstanding. For example, if there's a false rumor spread about him being a traitor. Another reason could be his sudden increase in power that makes his friends jealous. Maybe he accidentally harms one of them during a training session and they hold a grudge.
One possible reason could be a very different and perhaps unappealing take on Naruto's character in that fanfiction. Maybe the author made him overly cruel or out of character in ways that fans who love the original Naruto can't stand.
It could be due to a personal loss that has hit him really hard. Maybe he lost someone very close to him, like Jiraiya all over again, but this time it has broken him in a way that he shuts everyone out. He might also be under the influence of a new power that he doesn't fully understand yet, and this power is affecting his emotions, making him cold as a side - effect until he learns to control it.
In some fanfictions, it could be due to outside influence. For example, an enemy might use some kind of mind - control or false information to make Naruto's friends turn against him. Also, Naruto's own actions that are misinterpreted can lead to this. Say he tries to protect his friends in a way that seems wrong to them at first glance, like using a dangerous jutsu near them to fend off an attack.
Maybe in that fanfiction, Naruto's over - eagerness to be recognized could be misconstrued. His loud and boisterous nature might annoy people. For example, if he constantly interrupted others or pushed his ideas too hard without considering others' feelings, it could lead to dislike.
One possible reason could be envy. If Naruto suddenly gains a great deal of power or attention in the fanfiction, his friends might feel left out or jealous. Another reason could be a misunderstanding. Maybe Naruto is framed for something he didn't do, and his friends believe the false accusations without giving him a chance to explain. Also, in some fanfiction, a change in Naruto's personality, like becoming overly aggressive or secretive, could lead to his friends growing to dislike him.
Jealousy could also play a part. Naruto becomes very powerful over time. Some of his peers or even superiors in the village might be jealous of his growth and popularity. They could spread lies and manipulate others into believing that Naruto is a threat. This would gradually turn the whole village against him. For example, a less - known ninja who feels overshadowed by Naruto's achievements might start the ball rolling on the betrayal.
One possible reason could be that Naruto has had some really bad experiences that made him lose his confidence. For example, if he faced a series of powerful enemies alone and got severely beaten each time, it might make him scared of everyone. Another reason could be some kind of curse or dark magic that affects his mind, making him fear those around him.
One possible reason could be a powerful genjutsu cast over the village. Maybe an enemy ninja with strong illusionary powers managed to make everyone forget Naruto. It could also be due to some sort of curse. For example, a dark curse was placed on Naruto that made people's memories of him fade away gradually.