In the whole show each Eva went bersek at least once, and I figured out why.
I'll explain them in this order. Unit 2, Unit 0, Unit 1
This Eva is the easiest to explain because it only went berserker once. During the battle with the mass production Eva Series (MPES), the pilot of Unit 2 felt a great amount of rage. This in turn caused Unit 2 to go berserk. It seems that Eva's responds to rage; however, this only worked when the pilot of Unit 2 connected to it's soul.
This is by far the most misunderstood of all of them. It is widely believed within Unit 0 is the soul of Ritsuko's mother, this is wrong. That woman's soul is (along with her gray matter) within her prize creation the MAGI. If Unit 0 possessed a soul, it's either it's own or a fragment of Rei's soul. It is also believed that when Unit 0 went out of control it tried to kill several people; this I believe is false. If you follow Unit 0's moves carefully you will see what I mean. Both times Unit 0 held it's head as if in deep pain; and as most creatures do, it reacted to pain with violence. If you watched YuYu Hakausho during the dark tournament . One of the characters was in a great amount of pain and started pounding on the wall., much like Unit 0. It has also come to my attention that Eva may have also been afraid. Fear and pain can cause an Eva to go berserk.
This is by far the most complex of all the Eva's. In a number of occasions it has moved on it's own freewill. I believe there are two forces at work within it. One is the most common belief that Shinji's mother's soul is the cause. Though this is most likely true; I believe it is not the only reason. It is possible that he controls the Eva sub-consciously when he goes unconscious. It's possible his mother's soul acts as restraints for keeping him from going completely out of control.
Now a little bit about the A.T. Field.
The reason it is called the A.T. Field is because the A.T. stands for Absolute Terror. Absolute Terror is a mental condition that means creating a barrier around yourself to push others away; because of this, the light of the soul was called the A.T. Field by SEELE. Meaning, it was something to be destroyed.
This is Seele's lie.
By studying Evangelion , the book, the Anime and the game, I have learnt that this light of the soul should not be called an A.T. Field.
What it really is and what it's for, escapes me some what.
Yet, I do know this, It is not a field of Absolute Terror.