It's simple; human beings generally like following the crowd. No one will want to keep watching his profit sink below cost price when others are cashing out and market is dumping on you, except the long term investors.
So, it's just like a reflex action now....
One of the most common behavior of human being is that they do commonly follow the herd or the trend that is current happening.When they do saw that the price is changing or having some opposite direction and seeing their portfolio is going reds or negative then that is really a solid factor or reason which they would really
make out such action which is to panic sell.Cant really be avoided and even if you are experienced one doesnt mean that you cant really feel these emotions
and its inevitable but with due experience then you could at least make yourself able to do things which is a must.