It could be Vegeta. Maybe Vegeta does something really underhanded like stealing Goku's senzu beans or trying to take all the credit for a victory that Goku also contributed to. Goku, being a fair - minded person, would get angry at such behavior.
One common scenario is when his friends are in danger. Goku is extremely protective of his friends, and if he sees them being seriously threatened or hurt, he'll get mad. Another is when an enemy disrespects his family or the Saiyan race. Goku has a strong sense of pride in his heritage, and any slights against it can enrage him.
Well, perhaps in the fanfiction, Goku gets mad when he discovers that someone has been using his name for evil deeds. This would go against his values of justice and protecting the innocent. He might get really angry and go on a search to clear his name and stop the impostor. Or it could be that a new, extremely powerful villain emerges and starts to destroy planets without any remorse. Goku, being the defender of the universe, would be enraged by such senseless destruction and would confront the villain with all his might.
One reason could be ChiChi's overbearing nature. She is often very strict about Goku's training and how he spends his time. In fanfiction, this could be exaggerated to the point where Goku finally gets fed up. For example, if she constantly nags him about not doing enough chores around the house instead of training, it might push Goku to his limit.
It could be his rival. Rivals in Yu - Gi - Oh are always trying to one - up each other. If his rival manages to steal a rare card that Zane has been eyeing, or if the rival taunts Zane about a recent defeat, that would surely make Zane mad.
It could be a new, self - created villain. This villain might have been training in secret for years, specifically to defeat Goku. He could have studied all of Goku's moves and weaknesses and then launch a surprise attack that causes Goku to get hurt.
It could be because of a misunderstanding. In the fanfic, perhaps Annabeth was seen talking to an old acquaintance who Percy doesn't trust. He might jump to conclusions and get angry with her without really knowing the whole story. Like, if he thought she was plotting something behind his back with that person.
One possible reason could be a misunderstanding. Maybe Annabeth made a decision without consulting Percy, like choosing a quest path that Percy thought was too dangerous. Percy, being protective and having his own ideas about how things should be done, might get mad.
It could be because they have very strong expectations that aren't met. Maybe they were really invested in a certain story or character and were let down.