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.
In a 'yami gets mad fanfiction', it could be that Yami discovers a betrayal. Suppose he trusted someone deeply in a magical world they inhabit together, and that person has been secretly working against him. This realization makes him extremely mad, and he has to decide how to deal with this traitor while also protecting his friends.
In the fanfiction, Yami could be cursed by an enemy, which makes him sick. His strength starts to fade away. His friends notice the change in him and start to investigate. They discover the curse and search for a way to break it. Along the journey, they meet various characters who either help or hinder them. Yami's condition worsens, but his friends don't give up. Eventually, they find a powerful mage who can break the curse, and Yami is cured, learning valuable lessons about the power of friendship and the importance of being vigilant against enemies.
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.
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.
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.
Physical hurt might come from a powerful magic spell. Let's say an evil sorcerer casts a spell that drains Yami's energy and causes him pain. His struggle to overcome this and regain his strength could be a central part of the fanfiction.
One way to explore this in fanfiction could be through a duel. Maybe his opponent uses a new and powerful card that directly harms Yami. This could lead to his friends, like Yugi, rallying around him to find a way to counter the effect.
A common theme is protecting loved ones. When Trunks sees his family or friends in harm's way, he gets mad and is driven to take action. Another could be revenge. If someone wrongs him or his loved ones, his anger fuels his desire for revenge in the fanfiction.
It could be a new character, like a powerful wizard who has a grudge against Percy for some reason. Maybe Percy accidentally disrupted one of his spells, and the wizard is out to get him, which makes Percy mad.