Do you mean Transient ischemic attack, or mini stroke? There are many factors, it could be caused by:
A blood clot in an artery of the brain
A blood clot that travels to the brain from somewhere else in the body (for example, from the heart)
An injury to blood vessels
Narrowing of a blood vessel in the brain or leading to the brain
High blood pressure is the number one risk for TIAs and stroke. The other major risk factors are:
Atrial fibrillation
Diabetes
Family history of stroke
High cholesterol
Increasing age, especially after age 55
Race (African Americans are more likely to die from stroke)

Here is some useful information:
http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001743/