I can understand the extreme rise when woman earned the right to vote in 1920. Would have expect a greater rise in 1972 when 18 year-olds were given the right. I recall signs in windows of residential halls around campus in '72 calling for us to Pull Our Dick Out of Washington.
Seemed to be big increases Around 1929.......2008 after recession/depression? Later someone smart posted the women vote added in? Not sure when but probably back at the first big uptick. Then the youth vote? I had forgotten all of those factors, of course. and lazy.