One possible reason could be that he's constantly being underestimated or not given the credit he deserves. In some fanfictions, his friends might take his contributions for granted, which could lead to his temper flaring up.
In fanfiction, the author might explore the idea that Harry has a lot of pent - up stress from his past traumas. The constant reminders of his parents' death, the battles he has fought, and the responsibilities on his shoulders could all build up and cause him to lose his temper when something relatively small sets him off, like a minor criticism or a delay in a plan he has carefully crafted.
Maybe in the fanfic, Harry loses his temper when he discovers someone has betrayed his trust, like a false friend who leaked his secrets.
One possible reason could be bullying. If Percy is constantly being picked on by other students, it might push him to lose his temper. Maybe they tease him about his appearance, his family, or his abilities. Another reason could be academic pressure. If he's struggling with his studies and feels like he can't keep up, it could lead to frustration and ultimately a loss of temper.
It could be Draco Malfoy. Malfoy is always taunting Harry and his friends. His constant insults and attempts to undermine Harry might finally push Harry over the edge and make him lose his temper.
Perhaps he is betrayed by those he trusts. In the original series, there were instances of betrayal, and if in the fanfiction this is amplified, for example, if the whole wizarding community turns against him unjustly. He might feel so alone and hopeless that he loses his will to live. Also, if he discovers some dark secrets about his past that make him question his own identity and worth, it could lead to this state.
Another reason might be jealousy. If Harry suddenly gains more power or attention in the fanfiction world, his friends could become jealous. For example, if he discovers a new and powerful magic spell, and all the focus in the magical community is on him, his friends might start to feel overshadowed and distance themselves from him.
It could also be due to a misunderstanding. Rosalie might say or do something that Bella misinterprets. Say, Rosalie gives some advice about Bella's relationship with Edward, but Bella takes it the wrong way, thinking Rosalie is trying to interfere in a negative way and thus loses her temper.
In this fanfic, it might be that Percy is in a high - pressure situation like a really difficult quest. Things keep going wrong, his team isn't listening to his plans, and he just loses his temper out of frustration. He starts yelling at his teammates, accusing them of not taking things seriously enough.
It could also be due to a choice Harry makes that his friends don't agree with. Suppose Harry decides to take on a very dangerous mission alone to protect his friends, but they see it as him not trusting them. This could lead to a rift and eventually him losing their friendship.
Arthur could be the cause. He often bosses Merlin around and doesn't fully understand Merlin's capabilities. His constant demands and lack of respect for Merlin's intelligence might push Merlin to the edge. For instance, when Arthur makes Merlin do dangerous and pointless tasks without any regard for his safety.
Maybe his friends betrayed his secrets. In many fanfictions, a friend might accidentally or deliberately let out something Harry had confided in them, like his fears about Voldemort or his feelings for a certain girl. This could shatter his trust.