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.
Another character who could make Goku mad is Cell. Suppose Cell manages to come back in the fanfiction in a more powerful form and starts taunting Goku about his past failures. Goku's pride as a fighter and his desire to protect the earth would make him really angry at Cell's taunts and he would be eager to fight and defeat him once and for all.
Another possibility is a jealous rival. Maybe there's someone who is jealous of Gohan's power or his relationship with certain characters. This rival could do things to annoy Gohan and push him to the edge of his temper.
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 Perry the Platypus, in a really unexpected turn. Maybe Perry has to keep some secrets from Phineas for his spy work, and one day those secrets are revealed in a way that makes Phin feel betrayed. This would definitely make him mad.
A new player on an opposing team could be the cause. This new player might be overly cocky and disrespectful towards Kuroko during a game. They could constantly taunt him about his lack of presence, which finally gets on Kuroko's nerves and makes him mad.
One of his friends could be the cause, accidentally. Maybe a friend makes a deal with a shady character without realizing the consequences. This deal ends up putting everyone in danger, and Yami gets mad at his friend's naivete and the mess they've created.
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.
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 that Ron's actions were causing problems within the Order of the Phoenix. If Ron was being stubborn and not following the plans or orders set by the Order, which Sirius is a part of, Sirius might get angry. He would be frustrated with Ron for not being a team player and potentially endangering the mission and the lives of others.
It could be a female Namekian. Since Namekians are also very strong and have their own unique culture, a female Namekian could catch Goku's eye. They might bond over their mutual respect for life and their experiences with fighting powerful foes.
It could be a rogue Saiyan who wants to take over the Saiyan throne and thinks kidnapping Goku is the first step.