If a doctor was trying to rule out focal dystonia, why would he order a lumbar spine MRI if I said no back or leg pain or numbness or tingling?

Being thorough. Dytonia is usually due to dysfunction of the basal ganglia of the brain. Rare cases can be associated with focal pathology in the spinal cord. Often MRI will show some strange finding that is not clinically relevant. However, it will rule out a serious malformation or defect.