Why did interest in CS majors plummet in the United States after the mid-80s?
The Atari VCS game E.T. came out in December 1982, which led to the start of North American video game crash of 1983, the end of a video game software industry boom.  That market fell from 3.2B to perhaps 100M (in U.S. dollars) in just a couple years. Programmers were being laid off in droves.  Boxed software was being sent to landfill. Software and computer startups were starved for capital, or went under.  Game software developers lost much of their temporary "rock star" status.  That business event may have discouraged lots of kids from picking or sticking with a major where employment would have to come from a visibly crashed job market.

The dot-com stock market crash of 2000 (which also led to a vast number of computer industry related job lay-offs) probably had a similar effect on CS graduations after 2003.

Just after the '83 video game crash, easier-to-use personal computers (Mac, Amiga, Wintel, et.al.) were being introduced, which may have allowed end-user (business, science, etc.) productivity to increase faster than the number of software jobs previously required to support that productivity, thus making alternate (non CS) careers possibly more valuable, or more interesting to students, than CS related ones.

Added: The steep rise starting near 1980 is coincident with the PR leading into and through the IPO of Apple Computer (et.al.).  Computer magazines were becoming thick with ads.  Lots of kids wanted to either become a personal computer start-up company stock option millionaire, or at least work for one.

Added: The Netscape IPO (and PR about the millionaire programmers it created) in late '95 looks like it was a just a few years before a similar steep rise in CS graduation rates.