Is it their personality, an inherent pitfall of the trading profession or temporary insanity? A host of factor may contribute to a lack of discipline. Depending on your personality, background, training and experience with the market. You may have trouble reigning in your tendency to act on impulse.
That said, any trader can act impulsively at times. There are many situational factors that contribute to being impulsive. Research has shown, for example, that when people are tried, they have difficulty focusing their attention. As must as part of your conscious mind cares about sticking with your trading plan. Your unconscious mind thinks, who cares? I want to take a break. Psychological resources are limited. When you push yourself to the limits, you will have trouble focusing on your ongoing experience, concentrating on your trading plan and sticking on it.