If I remember correctly it is not at all related to fear but a disfunction in the basal ganglia (which performs action selection). Think of it as more like an inability to select an action or to select just one from a range of related movements (not at the conscious level, but between the conscious decision making and the signals being fired down your spine).

No, the pathology of Parkinson's Disease is dopaminergic dysfunction (the symptoms coming from this happening in the area relevant to planning movements, separate from areas that influence emotion or reward), usually driven by loss of dopamine-producing neurons. This is well-described by decades of research and tens of thousands of peer-reviewed papers.