It could be his rival. Rivals in Yu - Gi - Oh are always trying to one - up each other. If his rival manages to steal a rare card that Zane has been eyeing, or if the rival taunts Zane about a recent defeat, that would surely make Zane mad.
At first, their relationship might become strained. Zane's anger could make Syrus confused and a bit scared. Syrus might try to avoid Zane, not knowing how to deal with his anger.
In the 'aittic gets zane mad' fanfiction, it might be that aittic competes with Zane in a very unfair way. For example, in a contest or a game, aittic cheats but makes it seem like Zane is the one at fault. This would make Zane angry as he values fairness. Also, aittic could be constantly criticizing Zane's work or ideas without any valid reasons, which would also lead to Zane getting mad.
Maybe Zane gets mad when he loses an important Duel in the Yu - Gi - Oh world. It could be that he was so confident in his strategy but his opponent outsmarted him, leading to his anger.
One possible reason could be jealousy. Maybe Syrus is getting more attention in the fanfiction world, and Zane, being a proud character, doesn't like that. For example, if Syrus is praised for his skills while Zane feels overlooked, it can make him mad.