Based on the discussions of the experts at Quora, when you're in conditions of excessive emotion, stress will cause you to lose concentration. How to work your mouth not with nerve reflexes that are able to prevent you from unintended movement, but rather on what you think. So that the condition of your mind will be very influential.
If there is a myth that says that the cause you bite your lips part itself is someone cursed you or hit by bad luck, used to override that thought. However, even if it's just a myth in circulation, hence we will discuss the causes and the solution is scientifically.