One reason could be jealousy. For example, if one child in the picture story has a new toy and the other doesn't, the jealous child might lash out. Another reason could be lack of proper communication skills. They may not know how to express their feelings or needs, so they resort to hurting each other. Also, if they are imitating behavior they've seen, perhaps from an aggressive adult or a violent TV show, they might start hurting one another.
It might be due to competition. In the picture story, if there are limited resources like a favorite swing or a special treat, children may compete for it in an aggressive way. Or it could be that they are in a bad mood because of something that happened earlier in the story, like a scolding from a parent figure, and they take out their frustration on each other. Sometimes, children simply don't understand the consequences of their actions and think that hurting is a way to get what they want.
Educate them about kindness. In the context of the picture story, an adult or a wiser character could teach the children that being nice to each other is better. For example, through simple stories or examples within the picture story itself.
Jealousy can be a major reason. Just like in the story of two women fighting over a man. One woman might feel so envious of the other's relationship with the man that she is willing to fight to the death.
One possible reason could be a misunderstanding. Maybe Ginny overheard Harry saying something that she misinterpreted, leading her to want to avoid him. For example, if Harry was talking about his future plans and Ginny thought he was excluding her from them without really understanding his full intention.
One possible reason could be stress. In fanfic scenarios, she might be under a lot of pressure from her scientific research or protecting the Earth. The stress could build up to a point where she starts to harm herself as an outlet.
Snape could be distracted by his own dark thoughts or memories. For example, if something reminded him of his past with James Potter, his hatred for Harry's father might cloud his judgment for a moment and cause him to accidentally cast a spell in Harry's direction.
Perhaps Natsu is under the influence of a powerful and dark magic that makes him lose control of himself and accidentally hurt Lucy. It could also be that Natsu is trying to protect Lucy from something but his actions end up causing her harm instead. Like if he tries to keep her away from a dangerous situation by pushing her too hard and she gets injured.
It might be due to a lack of clear rules in their environment. If they are in a place where there are no strict guidelines on proper behavior, they may engage in such actions. Also, they could be trying to get attention in an improper way. For example, if they feel ignored, they might do something outlandish like this to draw focus to themselves.
They might hate each other because of differences in their personalities. Austin could be more outgoing and impulsive while Ally is shy and methodical. These differences might lead to clashes. For instance, Austin might make decisions on the fly that Ally doesn't agree with, causing friction. Also, jealousy could play a part. If one of them gets more attention or success than the other, it could breed envy and hatred.
Their personal feelings towards each other could also play a role. Leah might have felt that Jacob was getting special treatment within the pack. Jacob, in his eyes, might have seen Leah as too moody or difficult. So when they clashed, insults became a way to vent their frustrations. For instance, Leah could have said something like 'You think you're so great just because you're the alpha' and Jacob could respond with 'You're just a bitter person' or something similar.
One possible reason could be a sense of justice within a small community. If one daughter harms another, the mother of the harmed child might feel that the other mother should take stronger disciplinary action, which could be spanking. But this is more of a traditional view that is not so common today.