One reason could be a corrupt ruler. If the leader is stealing from the people, raising unfair taxes, or abusing their power, the common folk might form an angry mob to demand change.
In some fantasy novels, the discovery of a long - hidden secret can cause an angry mob. For example, if it is revealed that the ruling family has a dark and evil origin, such as being descendants of a demon, the people who have been loyal to them may feel betrayed. This sense of betrayal can fuel their anger and turn them into an angry mob. They may feel that they have been deceived and used by the rulers, and now they want justice.
One common theme is misinformation. Often in these fanfictions, the angry mob is formed due to false rumors or misunderstandings about the Avengers. For example, a false report of an Avenger causing harm to civilians can quickly turn the public against them.
Perhaps there is some kind of external force or situation in the story world that makes the hero realize the importance of the mob character. For example, the mob character might have saved the hero's life in a very subtle way that the hero only just discovered. Or maybe the hero has seen the true nature of the mob character during a difficult time, like seeing their selfless side or their strong will, and this has led to his sudden decision to propose.
Another reason could be if someone were to slander or disrespect his late mentor, like Iruka or Jiraiya. Naruto holds these people in high regard, and any negative remarks or actions towards them would likely enrage him.
Another reason could be related to school work. If there are new rules in Hogwarts that she deems unfair regarding academics, like restrictions on library access or new grading systems that don't reward hard work properly. Hermione is all about learning and fairness in education, so this could really make her angry.
One good 'Avengers Angry Mob Fanfiction' story is 'The Mob's Fury Against Loki'. In this, Loki's mischief has finally pushed the people too far and an angry mob forms against him. The Avengers have to decide whether to protect him or let the mob have their way.
One reason could be Boruto's recklessness. In fanfiction, Boruto might engage in dangerous stunts or actions that go against Naruto's teachings. For example, if Boruto were to use a forbidden jutsu without fully understanding the consequences, Naruto would be angry as it not only endangers Boruto but also those around him.
In a fanfiction, Natsu might get angry if he was wrongly accused of something. Say, if he was blamed for a crime he didn't commit in the guild or in the magical world at large. He's an honest person, and being falsely accused would be a huge blow to his pride. Another possible reason is if a magic - based experiment went wrong because of someone else's carelessness and it put his friends in danger. Natsu values the safety of his comrades, and such a situation would make his blood boil. Also, if someone were to try and break up the Fairy Tail guild by causing internal disputes, Natsu would be enraged as he loves the unity and spirit of the guild.
One possible reason could be that Tsunade makes a decision regarding the village that Naruto strongly disagrees with. For example, if she refuses to send help to a neighboring village in need, Naruto, with his strong sense of justice, might get angry. He's always been about protecting and helping others.