Another reason might be related to his friends. If the Order's actions put his friends in harm's way and he felt they were being careless about it, Harry would be angry. Maybe they used his friends as bait or made them do something against their will for the so - called greater good of the fight against the Dark Lord. Harry values his friends highly and any threat to them from the Order's actions would lead to his anger.
Well, perhaps in the fanfiction, the Order made decisions without consulting Harry. They might have put him in a dangerous situation without his knowledge, like sending him on a mission that seemed suicidal. It could also be that they were keeping secrets from him that he felt were important to his fight against Voldemort.
In fanfiction, Harry Potter might be angry if his friends are in danger and no one seems to be taking it seriously. He's very protective of his friends, like Ron and Hermione. So if some authority figures in the story are being dismissive of the threats his friends face, that would surely make him angry.
His anger could also cause a rift within the group. His friends might be caught in the middle, not knowing whether to side with Harry or the Order. This could lead to some tension and arguments among them. Like Hermione might try to reason with Harry, but Ron could be more on Harry's side, causing a divide.
One possible consequence is that he might distance himself from the Order. He could stop following their instructions or even leave the group if he feels that his anger is not being addressed or if he believes that they are not changing their ways.
Perhaps the wizarding government or authorities were being unjust. For example, if they wrongly accused Harry of a crime he didn't commit, his sense of justice would be outraged. His anger would drive him to fight back and prove his innocence. He's not the type to let false accusations stand.
A major reason could be differences in their approaches to fighting the dark forces. Maybe in the fanfic, Hermione wants to use a more diplomatic and academic approach, Ron is hesitant and just wants to follow the status quo, and Harry wants a more direct and action - based approach. Their inability to reconcile these differences can lead to Harry being angry. For example, when they are planning how to deal with a new dark wizard threat, their arguments and lack of compromise can make Harry frustrated and angry with them.
One reason could be for protection. The Order might think that Harry is in danger and they want to keep him close. So they summon him to a safe location where they can watch over him.
Well, perhaps in the fanfiction the Hogwarts staff might have been unfair in grading his exams. Like giving him lower marks than he deserved just because they had some sort of prejudice against him. It could also be that they wrongly accused him of something he didn't do, like misbehaving or cheating. Another reason could be that they failed to protect him or his friends from some sort of danger at Hogwarts.
It could be due to a misunderstanding. Perhaps someone spread false rumors about him, and as a result, people started treating him differently. He might be angry at everyone because he feels that no one bothered to find out the truth from him. Also, if there were some new rules or restrictions imposed on him without proper explanation, that could fuel his anger.
Another reason could be that Harry sees the Order as being too cautious or old - fashioned in their approach to fighting Voldemort. He might believe that their traditional methods are not enough to defeat Voldemort quickly and effectively, and this lack of progress makes him angry at them.
Maybe he feels that the Order is keeping secrets from him. In many fanfictions, Harry can be a character who values honesty and when he discovers that the Order, which he may consider an ally, is hiding things from him, it can make him angry.