This is not to say that anger is not real or normal. It is. And often justified. But anger comes second after a primary emotion comes first. Sometimes that primary emotion isn’t even recognized or realized because it is below the surface.
Imagine a tree, with its trunk and branches tall and large above the ground, and the roots under the Earth. What do those roots do for the tree? They feed it, right? So imagine the trunk and branches are the anger on your outside, the part that people see. But below the surface is the root of the anger, fueling it. If a student was walking down the hall at school and someone tripped him, he might jump up in anger ready to fight, angry. But right before that, he might have felt embarrassment. If a teenager is late for curfew and her parents have been pacing the house, when she walks in late they might argue and be mad and punish her. But right before that, they were worried. There can be any number of emotions below that surface that trigger anger, and depression is one of them.