You're probably referring to the emotional aspect of being impulsive and not greediness for it's our nature to have a sudden and unreflective urge to act if we find things in an instant is favorable to us without any second thought. Though I may say that greediness is the main element or the endmost factor that a trader loses their risk management but there are still some reasons why it always happens like being impulsive and having a less strategic plan that gives them a wrong response to any sudden changes on the market.

In the end risk management in trading can be considered as the best tool and could be the worse depending on how we handle it.