The exact cause as to why people develop Dystonia is currently unknown for the majority of people. For a small group of people it occurs due to a gene mutation, brain injury, infection, secondary disorder, or as a result of medication. Pinning down the root of the condition is something that research is currently focusing on.
So far we know that for some unidentified reason there is an issue with a section of the brain called the Basal Ganglia. It is known that this region of the brain enhances activity in the motor cortex which controls the agonist and antagonist muscles. In a healthy person when they make a movement the way the muscles contract and relax is coordinated and harmonious. However with Dystonia there is a deficient inhibition in the antagonist muscles which can result in both sets of muscles contracting simultaneously. It’s not clear why this happens.
The Dystonia Society UK have a fantastic wealth of information on the ins and outs of Dystonia, which I would really recommend reading to find out more information on the condition. For now it seems unlikely that any one particular theory will be proven right in the immediate future, so I shall continue to personify my Dystonia into a cheeky little alien, it’s a far more entertaining cause.