What causes a transient ischemic attack?

Blood clot. Usually a clot that occluded a blood vessel to the brain. It usually comes from a plaque on the carotid artery or the heart.
Blood clots. Typical tia's are associated with progressive closure of a blood vessel by thrombosis locally or by a clot at a distance lodging in a narrow blood vessel and blocking blood flow. The underlying risks include hypertension, heart disease, high blood fats (espec triglycerides), diabetes, and hyperhomocysteinmia) so to prevent a tia, treat underlying risks.