Researches from the University of California Los Angeles are examining the nexus between TBI and stress symptoms. According to the article, head injury researches have known about the link for some time but did not know how deep the connection went.
In extrapolating their findings, what they say is this: The physical trauma associated with a traumatic brain injury triggers material changes in a part of the brain that both regulates fear response and is highly vulnerable to physical trauma. Specifically, physical injury primes the amygdala brain region "for learning to be afraid."
Read the whole article here - http://www.palmbeachpersonalinjuryblog.com/2012/03/research-takes-step-in-examining-nexus-between-tbi-stress-symptoms.shtm