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.
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.
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, 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.
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.
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 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.
Perhaps an older teacher or coach who has always had an eye on Kuroko's potential could step in as a savior. They could see the signs of bullying and use their authority to punish the bullies and provide support and guidance to Kuroko, helping him to grow stronger both on and off the court.