What happens when you keep brewing coffee beyond a certain duration? It gets bitter, way too bitter to be consumed, it cannot be sweetened by any honey any more. Similarly, when a person holds onto their negative emotions, such negativity keeps on brewing, it makes the person bitterer. The longer they hold onto, the greater the bitterness.
Brewing anger can easily lead to a fit of rage because an outburst is more a symptom, an outcome, than a cause, it is a sign that you have been holding onto negative emotions within you. It is like heating up a steamed dumpling in a microwave oven. The dumpling is able to take heat to a certain degree, beyond which it simply explodes, splattering all over, it is no longer edible, much less presentable.
The brewer is a real killer, it is the slow poison. Unlike the volcano, this one is kept in heart. Many harbor anger and other negative emotions, they are unable to let go. ~Om Swami