One possible reason could be betrayal. If a teammate in the Young Justice team were to go behind his back or break his trust, Robin would get mad. He's a character who values loyalty highly.
Another reason might be related to his past. If something from his time as Batman's sidekick resurfaces in a negative way, like an old enemy coming back or a secret being revealed that makes him look bad. This could stir up a lot of anger in Robin as he tries to protect his reputation and deal with the new threat.
One reason could be mind control. Maybe an enemy has managed to gain control over Robin without the others knowing. Another possibility is that he has some hidden agenda related to his past. For example, if there are secrets from his training days that are now coming to light and making him act as a mole.
His anger might push his friends away at first. They might not understand why he's so angry and be put off by his sudden outbursts. For instance, if he snaps at Aqualad or Superboy, they could be taken aback and become a bit distant.
In a Young Justice Robin Cereal Fanfiction, the plot could revolve around Robin having a bet with his teammates about who can finish a large box of a really difficult - to - eat cereal first. But as they start the bet, they realize that there are strange things happening around the cereal. Like, it seems to be attracting small, strange creatures. So they have to stop the bet and figure out what's going on with this cereal before it causes more trouble.
One reason could be a difference in their methods. Robin might think Batman is too strict or old - fashioned in his approach to crime - fighting, and wants to do things his own way more freely. Maybe Robin has been influenced by new ideas or allies in Young Justice that Batman doesn't approve of.
It could be that Ron's actions were causing problems within the Order of the Phoenix. If Ron was being stubborn and not following the plans or orders set by the Order, which Sirius is a part of, Sirius might get angry. He would be frustrated with Ron for not being a team player and potentially endangering the mission and the lives of others.
Maybe there's a plot where they have to go undercover as a couple. As they pretend to be in love for the mission, they start to actually develop feelings for each other. The kiss could be a turning point where they admit their real emotions to themselves. For example, they might be infiltrating a villain's lair where only couples are allowed in, and during this time, their relationship deepens.
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.
Perhaps Percy forgets an important promise he made to Annabeth. In their relationship, promises are significant. If he fails to keep one, like a promise to help her with a project or be at a certain place at a certain time, Annabeth would surely be mad. It could also be that Percy accidentally breaks one of Annabeth's creations or possessions, which she holds dear.
Maybe Tails was tinkering with Sonic's shoes or some other personal item without permission. Sonic values his stuff and his independence. If Tails was messing around with his things in an attempt to 'improve' them but instead caused a problem, Sonic would likely be angry.
One possible reason could be if Naruto in the fanfiction is being unfaithful to her. In the original series, their relationship is built on trust and love. So, if the fanfic portrays Naruto in a way that goes against that, Hinata might get mad.