The growing popularity of “on the cusp”

The term cusp is an old one and it was first used in astrology and later in other contexts:

1580s, in astrology, "first entrance of a house in the calculation of a nativity," from Latin cuspis "point, spear, pointed end, head," which is of unknown origin. Astronomical sense is from 1670s, "point or horn of a crescent." Anatomical sense of "a prominence on the crown of a tooth" is from 1839. (Etymonline)

but the metaphorical expression on the cusp of something meaning “be at the time when a situation or state is going to change” became curiously popular only a few decades ago, from the 1990s as shown by Google Books.

While it probably derives from the astrological use of cusp, I wonder what may have caused its popularity which, as shown, has been increasing steadily and consistently in recent years.

Solution 1:

This ngram compares some apparently unrelated data. The timeline starts at 1960, and ends 2019. It seems to show an increase in many expressions relating to "on the cusp" starting roughly 1995, correspondingly with "Y2K"; and then increasing incrementally with my baseline ("on the way") , even though"Y2K" usage dropped significantly after 2001.

*ngrams started 2010, and only collects data until 2019. Perhaps it is due to a data collection bias.

Also interesting is that "Global Warming" dropped in mention by a significant percentage, even though "end of the world" crept up in usage, and Extinction Level Event never came close in any search...

I think "on the cusp" is just another expression that describes a certain type of morbid foreboding of a future change.

[I have seen past data that suggests that the majority of US citizens in the late 1930s knew there would be another great war in their then near future. But that would be out-of-scope for the question.]

My analysis:

This data indicates a fear of the future, and a reluctance to investigate the science.