It could also be due to false accusations. Some villagers who still hold grudges against the Nine - Tails might spread lies about Naruto endangering the village on purpose. The village elders, without fully investigating, decide to banish him to appease the angry villagers.
Perhaps an outside force is at play. An enemy village might use mind - control or other devious means to make Naruto seem like a traitor. They could manipulate the situation so that the Leaf village has no choice but to banish him, which would then weaken the Leaf village's strength.
One possible reason could be political intrigue. Maybe some powerful figures in the Hidden Leaf village see Naruto as a threat to their power. Since he is a Jinchuriki and has growing influence among the younger generation. They might engineer his banishment to get him out of the way.
At first, Naruto is likely to be confused and hurt. He has always considered the Leaf village his home and the villagers his family, so being banished would be a huge shock. He might feel betrayed.
In fanfiction, it could be due to a prophecy. Suppose there was a prophecy that Naruto would bring great danger to the village in the future. Tsunade, despite her bond with Naruto, might believe in the prophecy and banish him to prevent the foreseen disaster. Also, it could be related to Naruto's power. If his power was growing too rapidly and becoming uncontrollable, Tsunade might banish him to protect the village from any potential harm his power could cause.
One possible reason could be political pressure. Maybe some powerful clans in the village see Naruto as a threat due to his growing power and influence. They might force Tsunade's hand to banish him to maintain their own status quo in the village.
One possible reason could be fear. The villagers have always feared the Nine - Tails within Naruto. His family might banish him out of this same fear, not wanting to be associated with the potential danger he represents. Maybe they think that by banishing him, they can keep the village safe from the Nine - Tails' power getting out of control.
Perhaps Naruto was falsely accused of something really serious in the fanfiction. Like he was thought to be involved with the enemy in some way. Kushina, being in a position of power, had to banish him to maintain the order and trust within the village, even if she didn't truly believe he was guilty.
It could be the very people who banished Naruto in the first place. They might fear his new - found power and try to stop him from returning to the village or from getting revenge. They would likely use underhanded tactics to try and take him down.
One interesting plot could be Naruto going on a journey to find a way to clear his name. He might meet new allies who believe in his innocence and together they face various challenges.
One possible reason could be political. After the war, the villages might be restructuring and Naruto, with his great power, could be seen as a threat. Another reason might be related to the secrets he knows. Maybe some powerful figures don't want him to expose certain things. Also, his Jinchuuriki status could play a role. People might still be fearful of the power within him even after the war.
He could marry Hinata. Even though he's banished, Hinata might follow him because of her deep love for him. Their relationship could develop further outside the village, and they could get married and start a family.
Well, in this kind of fanfiction, Naruto might be banished because some powerful figures in Konoha fear his growing power. In Suna, he could discover a hidden power within himself related to the desert. He teams up with Temari and Kankuro to explore ancient Suna secrets. As he gets to know the people of Suna better, he begins to change their perception of him, and they see him as an asset rather than an outsider.