1.) Not always IN the brain per se. such chemical changes are also found in the periphery. Too much or too little can cause involuntary twitching. For example, in a state of having too much Calcium throughout your body, the muscles can contract/tense up (aka tetany). You can see this if you soak a disembodied muscle in a calcium solution.
2.) Also, not always a CHEMICAL problem, as a physical / anatomical abnormality in the brain or nervous system can cause uncontrollable movements. For example, people with Huntington's disease have structurally broken brain cells. When certain areas of their brain are affected, they have this uncontrollable neck twist motion that they do. The chemicals themselves may be normal levels, but the brain parts are messed up in a molecular level. There are other examples of larger structural problems causing seizures, like cancer, traumatic brain injury, strokes, brain tuberculosis or even worm larva in the brain.
3.) you can even have physical problems in the spinal cord or nerves outside the brain causing these problems. example: rabies causes involuntary motions as it affects nerve tracts connecting the brain to the muscles. 
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