I am not sure I agree with your statement regarding all adult defensive behavior.
First, even supposing that this is true, psychology tells us that memories of traumatic events are often repressed - sometimes so severely that they can never be uncovered. So even if this statement were true, can we ever say with absolute certainty that there was no trauma in child's life? I would say that every single adult has experienced some trauma during his/her childhood.
Childhood trauma could be physical and sexual abuse, but it could also be witnessing a car accident, a dead animal on the side of the road, a television character on television who gets shot and dies. Trauma does not have to be devastating or even recognized as traumatic by a parent sitting right next to the child.
Just think about your own memories from childhood. Why do certain events come to mind and others do not? Why does a sibling/parent/friend remember other events than we do? The answer is that each event a person experiences has a unique meaning for that person.
So taking your statement as true, it may just be that we cannot know the traumatic events that are causing defensive behavior in some adults.
If you would like, I can also discuss other ideas concerning why an adult may exhibit defensive behavior in the true absence of trauma or if we assume that childhood trauma does NOT cause adult defensive behavior.
Please let me know. Also, if there is a specific purpose to your question (i.e., a research paper versus a sibling/spouse/parent/child with difficult behavior) please provide me with more information so that I my answer will be the most helpful.
If you want to assume that it is from a person's past than the most common scenario is that the always-defensive adult has always felt a sense of resentment and jealousy toward others. Many individuals grow up feeling deprived, believing they deserved more than they received. Whether this is legitimate (as in a child who is raised in severe poverty) or not (as in the case of child who "has everything" but always wants what they do not have) is irrelevant. It is the belief that is important.
The atttitude that one is entitled to more than what one receives, than what life "gives" to a person tends to be a life-long attitude unfortunately.
With this attitude comes the expectation that everyone is going to deprive them of something to which they are entitled. Consequently, they become defensive based on this expectation, which they will tell you is a "fact" based on their lifelong experience. It is not necessary for anyone to actually deprive them of anything. Their defensivenes comes from their anticipation, from their certainty that they will be deprived.
In essence, people live their lives in defensive postures when their past experiences (or, more correctly, their interpretation/belief of their past experiences) has taught them that they will be deprived.
Please let me know if this answers your question.