Jeremiah (http://rhiever.github.io/name-age-calculator/index.html?Gend...) has an interesting rise in the 70s, but then disappears in the 80s (like so many things) but comes on strong again by the 2000s. The double humps doesn't seem to appear all that often, even clicking on the other examples in these HN comments, some of which appear to show spikes but are still normalish humps, not bimodal.
Another explanation, slightly more flattering to the parents, occurs to me: the popularity may owe something to the movie "Jeremiah Johnson", which came out about then.
What surprised me is that quite a few old-timey-sounding, probably-Biblical names ("Jebediah", "Ezekiel", "Jeremiah", "Abraham", "Noah") were either practically non-existent or otherwise rather unpopular 50 years (and further) back from the present.
