General Discussion

I think the writer was trying to show the pattern of "about every 50 years."

The labor movement in the U.S. began a little earlier than 1920, and there's no reason to focus on 1970 specifically. I think they were just counting off every 50 years leading up to 2020 and showing how close that timing is to regular eruptions. (One could also argue that 2016 was an eruption year and count back by 50 years and get closer to historic landmarks: 1966, 1916, 1866. Still not exactly right but close.)