There are various causes for dystonia. Among these are genetic or hereditary causes (runs in the family) and acquired causes from events occurring during ones life such as brain injury, infection or exposure to drugs or other toxins. However, most causes of dystonia remain unclear.
Dystonia is thought to result from an abnormality in or damage to the basal ganglia or other brain regions that control movement and abnormalities in the way the brain processes information and generates commands to move.
Please visit the following sites for more information:
Dystonia Fact Sheet (NINDS)