It could be that the Order has a plan that only Harry can be a part of. His unique experiences and skills, like his ability to resist the Imperius Curse and his connection to Voldemort, are valuable. So they summon him to put these to use in their fight against the Dark side.
Well, he might be given a dangerous mission. Maybe he has to infiltrate a Death Eater stronghold or protect a crucial magical artifact.
Maybe he is summoned to receive special training. The Order could have found a new way to enhance his magical abilities, like teaching him some ancient and powerful spells that can be used against the Dark Arts. It would be a great opportunity for Harry to grow stronger.
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.
Maybe for the sake of an interesting crossover. Writers often bring different characters from various universes together to create unique and exciting stories.
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.
Maybe he has read some really bad Order fanfictions before that were full of inaccuracies. For example, the characters' personalities were completely distorted or the plot was so far - fetched that it made him lose trust in all Order fanfictions.
The order of the Harry Potter novels is important because it tells a continuous and evolving story. Each book builds on the previous one, developing the characters, relationships, and the overall plot.
In some fanfictions, Harry Potter might be angry at the Order because he feels they are keeping secrets from him. He is used to being in the thick of things and when the Order withholds information, it can make him feel left out and untrusted.
Maybe in the fanfiction, the Order has some actions that Harry disagrees with. For example, they might be too controlling over his life and decisions. Harry is a very independent person, and if the Order tries to restrict his freedom in the story, it could lead to his hatred.