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.