One possible scenario could be when Merlin is constantly underestimated in Camelot. People might make fun of his clumsy appearance or dismiss his ideas. After a series of such incidents, he loses his temper. For example, if Arthur repeatedly ignores Merlin's warnings about a coming danger and then blames Merlin when things go wrong, Merlin might snap.
Another situation could be related to the treatment of magic users in Camelot. If Merlin witnesses the unjust persecution of an innocent magic user, especially a friend or someone he feels responsible for, his anger might boil over. He has been trying to keep magic a secret while also protecting those with magic, and seeing such injustice could make him lose his temper.
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.
One possible scenario could be that Aang loses control of his Avatar state and causes destruction in a village. He might be under extreme emotional stress, like the loss of a dear friend, which triggers this loss of control.
Perhaps in the fanfiction, Harry loses hope when he is betrayed by someone close to him. It could be a friend from Hogwarts who turns on him for personal gain or out of fear. This betrayal makes him feel isolated and like his fight against evil has no meaning. He might then start to withdraw from the fight, stop using his magic, and become a shell of his former self.
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.
Perhaps there's a magical mishap. A spell gone wrong could cause the scarf to disappear. It might be a spell that was aimed at Natsu but instead of harming him directly, it targets his most precious item, which is his scarf. This could lead to a whole adventure where Natsu tries to get his scarf back, facing various challenges along the way.
One possible scenario could be that Gwen is in danger from a powerful dark magic user. Merlin steps in to protect her, knowing that the only way to defeat the enemy is to sacrifice his own life force or magic. Another could be that Gwen is wrongly accused of a crime she didn't commit, and Merlin sacrifices himself to prove her innocence and save her from execution.
He might accidentally reveal his identity. In some fanfictions, when Peter Parker loses his temper, he can't control his actions or words as well. This could lead to him slipping up and showing his superpowers in front of the wrong people, which would put his secret identity at risk.
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.
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.
His own self - doubt could be the cause. Percy might be constantly thinking that he's not good enough to protect his friends or complete his quests. And one small trigger, like a friend making a harmless joke about his capabilities, could make him lose his temper because it hits too close to home.