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.
I haven't actually read this specific 'piccolo and gohan lemon fanfiction', but generally, in fanfictions involving these two characters, it might explore their relationship in unique ways. It could be about their friendship evolving, or perhaps their teamwork in new and unimagined scenarios within the Dragon Ball universe.
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.
I'm not entirely sure which specific 'piccolo x gohan lemon fanfic' you're referring to. But generally, if it's a fan - made story, it might explore unique relationships and character developments. It could potentially show how Piccolo and Gohan interact in a different way than in the original series, perhaps delving into their friendship or a more complex emotional bond.
One great fanfiction could be about Gohan and Piccolo's training sessions. Gohan, with his Saiyan - human hybrid power, and Piccolo, the Namekian, could be exploring new ways to increase Gohan's power. Maybe they discover a new technique while sparring in the wilderness. This would not only strengthen their bond as mentor - student but also as friends who rely on each other in battles against stronger foes.
Training is also a popular theme. Since Piccolo was Gohan's mentor for a while, fanfics often show them training together and Gohan achieving new levels of power with Piccolo's guidance. There are also themes about family, as Piccolo becomes like an honorary family member to Gohan. He protects Gohan and is there for him in a way that is similar to a family member, and this aspect is often explored in fanfics.
One possible theme could be growth. Gohan has always been on a journey of growing up and Piccolo has been part of that journey. In the fanfiction, this growth could be explored in a more personal and emotional way, perhaps related to their relationship.
One possible theme could be friendship. Given that Piccolo and Gohan have a long - standing relationship in the Dragon Ball series, the fanfic could further explore the depths of their friendship, how they support each other through different challenges.
It could be that a new threat emerges. Maybe another android, stronger than Cell, was observing the fight and decides to take advantage of the chaos after Gohan's death. This new android could become the main antagonist as it poses a greater danger to the remaining fighters.