What causes cluster headaches?

Exact cause unknown. In some cases, there is a strong genetic component, but this is not consistent. Histamine is thought to play a role; injection of histamine provokes attacks in 69% of patients. Stress, allergens, seasonal changes, or nitroglycerin may trigger attacks. Risk factors include: male sex, cigarette smoking, and previous head trauma.
Cluster headaches. As w/most types of headaches (aside from infection, excess alcohol or caffeine withdrawal), we have no idea why someone suffers from one type or another, including cluster. Check out http://www.mayoclinic.org/diseases-conditions/cluster-headache/basics/causes/con-20031706 for more info.