One possible reason could be that they had a past traumatic experience related to explosions. Maybe they witnessed an explosion that caused harm to their loved ones or themselves. Another reason might be that explosions in the 'Naruto' world are often associated with powerful and dangerous jutsus or attacks, and they are simply afraid of the destruction and chaos that explosions can bring.
Well, perhaps they grew up in an environment where explosions were rare but always meant extreme danger. So, when they encounter explosions, their natural instinct is to be afraid. Also, in the Naruto universe, explosions can be a sign of a very powerful enemy. Kiba and Isse, if not strong enough to handle such powerful attacks, would of course be afraid of explosions.
One possible plotline could be that they are in a village or a world where there is a sudden increase in the use of explosive jutsus or weapons. Naruto, Kiba and Issei have to face their fear as they are constantly under the threat of these explosions. Maybe they start off by trying to avoid areas where explosions are likely to occur. For example, they might stay away from battlefields or places where ninja experiments with explosive substances are taking place. As the story progresses, they could be forced into situations where they have to confront their fear directly, like when their friends or loved ones are in danger in an explosion - ridden area. This would be a great opportunity for character development as they learn to overcome their fear for the sake of others.
Another way to depict the fear is through their internal thoughts. The author could write in the first - person perspective for each character at times. Naruto might think about how the explosions remind him of the destruction he saw during the Pain invasion and how he doesn't want to see that kind of destruction again. Kiba could be thinking about how his ninja skills might not be enough to protect himself and his dog from the explosions. Issei might be having flashbacks to a past experience where he was nearly killed by an explosion and how that fear has now resurfaced.
Naruto could use his explosive - natured jutsus like the Rasengan. If he was sparring with Kiba, a misdirected Rasengan could hit something explosive nearby and cause an explosion involving Kiba. For Issei, if they were in a battle together, Naruto's chakra - infused attacks might react with Issei's energy - based powers in an unexpected way and create an explosion.
One reason could be for the sake of creating drama. Fanfic writers often like to explore extreme scenarios that deviate from the original story to generate intense emotions and reactions from readers.
One reason could be a past bad experience. Maybe he had a really painful treatment as a kid that left a deep mark. For example, if he had a serious injury once and the doctor was rough while treating him, it would make him fear doctors.
It could limit their growth as ninjas. Since they are afraid of explosions, they may shy away from battles or training scenarios that involve explosive elements. This would prevent them from learning how to deal with such threats effectively and thus hold them back in terms of becoming stronger ninjas. However, on the other hand, it could also drive them to find alternative ways to fight and protect themselves that don't involve facing explosions directly.
Perhaps Naruto has a newfound confidence or a change in his personality that makes Kiba and Issei twitchy. In the fanfic, if Naruto starts acting more recklessly or assertively than usual, it might make them nervous. It could also be that Naruto has some sort of connection to an enemy or a dark force that Kiba and Issei sense, and that's what is making them twitchy.
One possible scenario could be that Naruto, with his shadow clone jutsu, creates a diversion that leads to an accidental explosion involving Kiba and his ninken. As for Issei, perhaps in a crossover setting, Naruto's Rasengan clashes with one of Issei's attacks and causes an explosion. However, this would require a lot of creativity to blend the two very different universes in a believable way.
One possible reason could be past trauma. Maybe he had a bad experience in his childhood, like being bullied or physically hurt by someone. Another reason might be related to his magical abilities. He could be afraid that being touched might somehow disrupt his magic or cause an unexpected magical reaction.
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.