It is associated with difficulty with attachment very early in childhood/infancy, originating in events we have no memory of. How we did, or didn't, for attachments to our primary caregivers (usually Mom). This is when we learn that the world is a warm and loving place to be, that we are not dangerous and others are not dangerous, that is okay to just be.
Secure attachment can form, then be disturbed by trauma.
Or, secure attachment may not form, for some reason (Mom is not well, child has to be separated from mom for some reason like illness, etc.)
Or, the attachment can seem to be secure, but Mom is overly attached, which can leave the child confused and afraid of separating, which is a normal process after infancy.

There are many many good books on attachment