The population of Descartes in central France increased by upwards of 150% from 1962 to 1968 after a general negative trend over the preceding seventy years. Why was this? All I can find is that the town changed its name from "La Haye-Descartes" to simply "Descartes" in 1967. I'd assume it has some relation to the population growth, probably a result of a small town of ~1,700 being flooded with ~2,600 new residents.
Based on this page from the French School for Advanced Studies in the Social Sciences, it appears that the neighbouring Balsesmes merged into La Haye-Descartes in 1966, before the combined commune was renamed to Descartes the next year.
In 1962, the two communes had remarkably similar population levels of 1,679 and 1,689. With a combined population of 4,267 in 1968, the growth rate was only 26.7%. Still high, mind you, but not the shockingly stratospheric 154% that it initially appears.