Alzheimer's is primarily a disease of old age, so where you have an aging population you'll see it increase without any change in the risk factors the population's exposed to (some of which I believe are genetic, it seems to "run in families", don't ask me for a reference..).

If one in n people will have developed Alzheimer's (or any another disease) by the time they reach (say) 70, but 1 in 3n by the time they reach (say) 75, and the life expectancy of the population goes from 72 to 77, disease rate will go up sharply (I made up the numbers to furnish the explanation, I've no idea how fast Alzheimer's rates go up with age, only that they do..)