Another interesting potential cause or contributor being researched is BMAA produced by blue-green algae and various other sources.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3295368/ [2012]

http://rspb.royalsocietypublishing.org/content/283/1823/2015... [2016]

It's really hard for a medical layman such as myself to sift through all the science. This source is interesting for the story of its discovery via ethnobotany even if it doesn't end up a smoking gun cause of Alzheimer's or other neurodegenerative diseases.