There are plenty of things that can lead to Parkinson's. Recently we learned even copper salts carry a risk: https://www.sciencedirect.com/science/article/abs/pii/S01618...

(Unfortunate for many like me who considered them relatively safe for years and did a lot of chemistry with them.)

Farmers use many chemicals for a variety of tasks and it wouldn't surprise me if there were multiple chemicals involved, perhaps even synergistically. Maybe a farmer exposed to paraquat is fine, but one who is exposed to both paraquat and a copper-based antifungal or fumes from a welding repair become more damaged. Hard to say.