Genetics and probably severe stress. Mood disorders rampant on both sides of my family and schizophrenia on one side (an uncle) , I have schizoaffective bipolar type. My first episode was severe depression at like 14 years old but I wasn't dx'd with bipolar/schizoaffective until 18/19 after psychosis and manic episodes. I grew up homeless so that was extremely stressful so I guess I had it coming from both angles, genetic and stress wise and it really set off as a young adult which is pretty typical I guess