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.
Maybe he gets mad when his friends are in danger. For example, if Videl is threatened by a powerful villain, Gohan's protective nature kicks in and he gets really mad.
Maybe Chi - chi is constantly comparing Gohan to Goku in the fanfiction. Gohan has his own identity and abilities, and being compared all the time can be frustrating. He could get mad because he wants to be recognized for who he is, not just as Goku's son. Also, if Chi - chi is interfering too much in his training or friendships, that could also lead to his anger.
It could be a new, unknown enemy. Maybe someone from a far - off planet who has a grudge against the Saiyans. This character could be really powerful and sees Gohan as an easy target at first, so he throws a punch.
There could be. Fanfiction is a creative space where all kinds of wild ideas are explored. However, this particular concept seems rather unorthodox considering Gohan's relationship with Bulma in the original Dragon Ball series. Gohan has always seen Bulma more as an aunt - like figure. But in the world of fanfiction, anything is possible. It might be difficult to find though, as it may not be a popular or widely - accepted concept.
There could be. Fanfiction writers often explore different scenarios for their favorite characters. But keep in mind that these are not canonical. They are just the creative interpretations of fans who want to see different relationship developments for Gohan and Videl.
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.
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.
He might become withdrawn at first. Since he's used to being in control and having a certain dynamic in their relationship, Bella's anger could shock him into pulling back. He could go off on his own for a while to think about what he did wrong.