One possible reason could be a threat to Konoha. If Iwa planned an attack on Konoha, Naruto, as a defender of his village, might take the fight to them and end up destroying parts of Iwa in the process.
It could also be due to a personal vendetta of someone in Iwa against Naruto or his friends. If they made a move on Naruto's loved ones, he would retaliate and his power could be so great that it results in the destruction of Iwa.
One possible reason could be a threat from Iwa to Konoha. If Iwa was planning an unprovoked attack on Konoha, Naruto might take it upon himself to destroy Iwa to protect his village.
Sasuke could be involved. He and Naruto often have intertwined stories. Sasuke might either be an ally in Naruto's destruction of Iwa, or he could be there for his own reasons, like seeking power or revenge.
There might be characters from Iwa who oppose Naruto. For example, the Tsuchikage of Iwa would surely be involved as the leader of the village Naruto is attacking.
Maybe in the 'naruto neglected iwa fanfiction', there's a story where Iwa has a secret technique that could have been useful in the great ninja wars, but because they are neglected, they are not invited to share this knowledge. As a result, a young Iwa ninja goes on a journey to find a way to make the other villages recognize their worth and the importance of their techniques.
Naruto is obviously a very popular character in these fanfictions as he is the main character of the original series. In Iwa, characters like Deidara (who was originally from Iwa) often make appearances. His explosive jutsu and unique personality add a lot to the stories.
One great Naruto Iwa Admiral fanfiction is 'The Stone Admiral's Ascent'. It has an interesting plot where the Iwa Admiral character shows unique jutsu and strategic thinking. It really delves into the character's backstory and how they interact with the Naruto universe's existing elements.
Maybe he feels ostracized in Konoha. If for some reason the villagers still don't fully accept him despite his heroics, he could be lured by the idea of a fresh start in Iwa.
Naruto could meet some young and upcoming Iwa ninja trainees. They might be curious about Naruto and his abilities from Konoha. These trainees could either look up to Naruto or be jealous of his fame, leading to various interactions. Also, there could be some old - timer Iwa ninjas who remember past events involving Konoha and have their own views on Naruto's presence in Iwa.
Another way is that he forms an unexpected alliance within Iwa. There could be a dissident group in Iwa who oppose the capture of Naruto. They might help him escape in exchange for some future favor or because they believe in Naruto's cause. Naruto has a way of inspiring people, even those who are initially his enemies, so this is a plausible scenario.
It could be that Kiba has been involved in something that Naruto considers wrong. For example, if Kiba endangered the safety of the village or their friends, Naruto, being the protector - type, would take action against him. Or perhaps there was a misunderstanding between them. Kiba might have been framed for something, and Naruto, believing the false information, tries to 'destroy' Kiba in the heat of the moment.