the punching bag

There are some things you cannot fight, and the frustration builds until you let it out. Often, we take it out on others.
It is obviously human nature, easy to spot until we are doing it, in which case our behavior is valid and justified.
When you are in a fight, your focus narrows, we become myopic.
Furthermore, humans are centrifugal in their bodily reactions: the more intense the pain, the more the effects spread out (e.g., a wound on your finger affects your whole arm, not just the localized spot).
Consider that the anxiety and anger bleed into a full body reaction. Yet, when let out on others, we focus like a laser, losing perspective and context, losing ourselves to the action.