There are no known direct causes of Psychotic Illness however there are a range of things that are known to be risk factors. As with most mental illnesses a diathesis-stress model is used to explain how psychotic illness develops. A diathesis-stress model basically argues that illness develops due to a combination of genetic vulnerability and exposure to specific risk factors. So, if a person happens to have a vulnerability to becoming psychotic and certain risk factors are present then the illness develops. An example of this would be when a person with a family history of Schizophrenia (and therefore genetic vulnerability) uses Marijuana heavily (a known risk factor for Psychotic Illness) and becomes unwell.
Some of the known risk factors for Psychotic Illness are:
Extreme psychosocial stress
Mood disorders (when mood disorders become very severe they can trigger Psychotic episodes)
I hope this has been of some help. If you have further questions or would like me to clarify any part of my answer please let me know.