It could be Perry the Platypus, in a really unexpected turn. Maybe Perry has to keep some secrets from Phineas for his spy work, and one day those secrets are revealed in a way that makes Phin feel betrayed. This would definitely make him mad.
Often, in fanfictions where Hiccup gets mad, there's a plotline about him getting angry with his father's old ways being forced upon him again. Maybe the elders on Berk want to go back to the old dragon - fighting traditions, and Hiccup is furious because he has worked so hard to create peace between dragons and humans. He might have to fight against this in a very angry and determined way. Additionally, he could get mad at someone who mistreats a dragon he has befriended, which would trigger his protective side and his anger towards the wrongdoer.
One idea could be that Ron has been having a bad day and seeing Hermione at the Quidditch match makes him snap because he feels she should be studying instead. Maybe he's worried about upcoming exams and thinks Hermione is being too carefree. Another idea is that Ron is jealous. He sees Hermione chatting with one of the Quidditch players and gets mad because he secretly has feelings for her but doesn't know how to express it.