Anger in your child is due to the build-up of different stress-creating situations that is happening, or has happened in the past. Your child become angry in different situations like if their peers jeer or tease them, or if they compelled to do something not to their liking, or if others push or hit them, or if somebody takes away their things, or if their peers refuse to mingle with them, and such.
After the initial arousal of anger, different children find different ways of expressing their anger. Children form different ideas about expressing their anger.