It is astonishing, isn't it? I'm reading Sirens of Titan right now and luck is such a big theme in it. You can always increase your odds by healthy living, but nothing is guaranteed. It really comes down to dumb luck.
Let's not forget Ozzy's immense wealth that allowed him the best medical care and health/fitness programs in his later years. He stopped living "hard" by the 2000s, at least by his 1980s standards.
Drug use is a risk factor for Parkinson's. A lot of drugs affect dopamine production or reuptake, and that seems to be able to cause a reduction in dopamine production later in life.
Some of the symptoms (like tremors) of withdrawal after long term stimulant/cocaine/amphetamine use are similar to parkinsons too since both cause a shortage of dopamine in the brain. I kinda wonder if one could accidentally mask early symptoms of parkinsons.
Given that Parkinson's is a disease in which the neurons in the brain stop producing dopamine, it's just as possible that his drug use was treating his condition, not worsening it.
From what I can tell, Parkinson's doesn't have a direct, established environmental cause. Just some candidates which may increase risk.
Note: I'm not an MD or have any background in medicine.