The term 'Cause of an Outburst' can be summarized by two crossword solutions: ANGER (5 letters) and Last Straw (9 letters). ANGER is a strong emotion that is often associated with an outburst. It is usually caused by a feeling of frustration, hurt, or injustice. The phrase 'last straw' refers to the idea that a person has reached their limit and can no longer tolerate a situation. This phrase is often used to describe the moment when a person's ANGER reaches its peak and they finally lash out.