Another possibility is that he could be afraid of his own power. In certain fanfiction scenarios, Naruto might discover a new, uncontrollable aspect of his chakra or a hidden power within him that he doesn't understand. The fear of accidentally harming others or losing control of himself could make him afraid.
One reason could be the fear of losing his friends. Naruto has always been very protective of his comrades. In some fanfictions, he might face a situation where he is on the verge of losing them, like a powerful enemy threatening to kill them all. This would make him afraid as his friends are his most precious things.
He may also be afraid of failing his village. Naruto has a great sense of responsibility towards Konoha. If there are threats in the fanfiction that he feels are beyond his ability to handle, such as a large - scale invasion that he doubts he can repel, it could instill fear in him as he doesn't want to let his home down.
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.
One possible reason could be that he has had some extremely traumatic experiences with certain ninjas. Maybe he witnessed a powerful ninja going on a rampage and killing innocent people, which left a deep scar in his mind, making him fear ninjas in general.
In some fanfictions, Naruto might be afraid of doctors because of past traumas related to medical treatment. Maybe he had a bad experience during a recovery process, like a painful treatment that left a psychological mark.
Perhaps in the fanfiction world, there is some sort of magical or technological device that mutes Naruto. This could be part of a plot where an enemy uses this device to stop Naruto from using his inspiring speeches or to isolate him from his friends. By muting him, they can disrupt his normal way of communicating and leading, which is a key aspect of his character.
Maybe he's being punished for some misdeed in the fanfiction world. For example, if he accidentally broke an important village rule or caused some chaos during a mission. Another reason could be that someone is trying to keep him safe from an outside threat. They might think that locking him in his house is the best way to protect him from enemies who are after him.
In some fanfictions, Sakura may make decisions without considering Naruto's opinion. Naruto values his friends and their input, so if Sakura goes ahead and does something like endangering the team or making a big decision on her own that affects everyone, Naruto could get mad. Also, if Sakura were to insult Naruto's dream of becoming Hokage, that would definitely be a reason for his anger as that dream is very important to him.
One possible reason could be the rich and diverse world of Naruto. The complex characters, unique jutsus, and the overall ninja theme are very appealing. Fem Kyuubi might be attracted to the different interpretations and new storylines that fanfiction offers, which can expand on the original Naruto universe in creative ways.
Well, it could be that Naruto's newfound fear makes him hesitant in battles. He might struggle to use his usual jutsus because he's afraid of the potential consequences. For example, he could be afraid that his Rasengan might cause more destruction than intended and harm innocent people. So, in battles, he'd be constantly second - guessing himself, which would make for an interesting plot as his friends try to figure out what's wrong with him and help him overcome this fear.
One possible plot is that Naruto accidentally awakens a memory or power related to Kushina that is so overwhelming and intense that it makes him afraid. He then has to deal with this new - found fear while also trying to understand his mother better. It could also involve him having to face Kushina in some sort of spiritual or dream - like encounter and being terrified.
It could be due to the failure of protecting his village or his loved ones. Naruto has always been about protecting those important to him. If he fails at that, his anger is understandable.