Parkinson's probably does not start in the brain, or even in dopamine neurons. Whatever kicks it off happens in other places like the gut, the spine, or other less central locations. The presence of alpha synuclein aggregates as neurite filaments and Lewy bodies in places like the vagus nerve, for instance, years before their appearence in the basal ganglia convinces me of this.

My money is on an inflammatory trigger that activates macrophages/microglial cells of the endogenous immune system to produce superoxide and other ROSs (reactive oxygen species). It is likely to initially be an over-reaction of an otherwise normal immune response to some substances that are mistaken for a foreign invader; something like what Heidi is referring to in gluten sensitivity, but not always involving the formation of antibodies by the adaptive immune system.