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.
In the fanfiction, it might be someone from his own team. Maybe there's a new member who is insecure and feels that Kuroko is getting too much attention. So, in a fit of jealousy, this person does something that causes Kuroko to get hurt, like tampering with his equipment.
Well, a frequent theme in 'angry kuroko fanfic' is the breaking of his calm exterior. Usually, Kuroko is so calm, and when he gets angry, it shows a contrast. There's also the theme of justice. His anger might be a result of seeing injustice in the basketball court, like unfair refereeing or opponents using dirty tactics. And then there's the theme of growth. Through his experiences of being angry, he may grow as a person and as a player.
Well, perhaps in the fanfiction, someone could have insulted Kuroko's basketball skills. Kuroko, usually so calm and reserved, gets mad and decides to show off his real capabilities on the court in a very intense and uncharacteristic way. He might start trash - talking his opponents, which is something he never does normally.
Well, it could be about Kuroko from a certain anime or manga obtaining the 'emperor eye' ability. This might lead to him having enhanced perception on the court in a sports - related context, like seeing opponents' moves before they happen and making better plays.
Well, in the 'kuroko gets sick fanfiction', perhaps his friends would take care of him. They might bring him soup and stay by his bedside to keep him company.
Well, sometimes in these fanfictions, Kuroko might get angry when his plans on the court are constantly disrupted. He's a strategic player, so if his teammates don't follow his lead or if the opponents manage to anticipate and counter his every move, it could push him over the edge. Also, if there's a situation where the team is about to lose because of someone's negligence, Kuroko's anger might flare up as he has a strong desire to win and see his team succeed.
Danzo could also be the one to make Naruto angry. Danzo's underhanded tactics and secrets might be exposed in the fanfiction, and if they were to directly harm Naruto's friends or the village, Naruto would be furious at his actions.
After Natsu gets angry, the plot might develop with other guild members trying to mediate. Happy might try to calm Natsu down while Gray could be in the middle trying to make both sides see reason. This could create a lot of comical and tense moments in the fanfiction.