It's multifactorial. If you read about the biopsychosocial model you can break down some of the components that all contribute to health or illness, mental or physical.

The way I understand most psych conditions, we are all born with particular dispositions and potentials and sometimes vulnerabilities. The way that we are raised also contributes to our growing personalities and the strengths and weaknesses that we develop. If we are vulnerable, and something happens that triggers a particular vulnerability, and especially if we don't get the support to deal with that vulnerability well, we can become mentally ill.

I think that if you look hard enough, most psych conditions could be linked to some kind of trauma at some point in life. But trauma doesn't have to result in mental illness either. Resilience and support and early treatment can go a long way.

Sometimes the source of abuse or trauma goes unrecognized. But it's still what it is, even if hidden.