What are the possible causes of idiopathic subarachnoid hemorrhage?

Do not know: By definition idiopathic means 'we don't know' so we won't know what causes idiopathic subarachnoid hemorrhage.

Vascular defects?: Subarachnoid hemorrhage is bleeding into the fluid around the brain. Trauma and hypertension can be a cause. In spontaneous cases one must consider a vascular malformation such as avm or aneurysm. A good quality MRI would be appropriate. Discuss this with your doctors.