Even if a condition is pre-existing, the injury can always cause an exacerbation of the problem. Injury is a stressor. So if someone has always been depressed, it would appear that an injury, by definition, is then a contributing cause to the depression after injury.
That is actually not correct. There are many forms of depression as explained previously. Some are related to the current situation and are called adjustment disorders.
However, there are some depressions that are the result of genetic and familial patterns. These very often occur in families that have histories of other mental disorders ranging from depressed relatives, to schizophrenic and addicted family members.
These chronic and recurring depressions are going to occur independent of external events. However, the patient will try to attribute everything to an injury either as a form of malingering or due to poor understanding of their own psychological condition.
A careful history will reveal whether there has been a long (often never formally diagnosed) history of depression. That depression will have presented itself over and over and led to very maladaptive life patterns and often past suicidal attempts. The individual may have been medicated with little if any change. They may have seen numerous doctors for vague complaints that suggested, but may not have been diagnosed as, depression.
Injury becomes a way for the patient to explain the most recent dysregulation in mood. It is a grave disservice to the patient to attribute a chronic problem to a recent event since it may block the patient from seeking care for what may be a lifelong problem.
Just because someone has not been in psychological care does not mean that they have not long suffered from depression. And just because someone is said to have successfully completed psychological care does not mean that they may continue to suffer from depression but present the depression as physical complaints.