I‘m not sure I‘m buying the prion theory. Shouldn’t numbers decrease as people go outside ever less often? Prevalence of ALS seems to be increasing though: https://www.ncbi.nlm.nih.gov/labs/pmc/articles/PMC6735526/
I think the Borrelia Burgdorfferi hypothesis would fit better, with Lyme disease numbers climbing in recent decades.
Possibly infection when being looked at for a head injury, but I assume they take decent precautions.
Another user proposed grass was the commonality between those sports, in that case grass/dirt entering the eyes?
For forest/streams, possibly water/dirt entering the eyes?
Doesn't really narrow much down though, avoid all dirt, grass, and water.