My imagination? Here it says there were two substantial series breaks (1990, 1994): http://actrav.itcilo.org/actrav-english/telearn/global/ilo/seura/usunemp.htm
And here we get a sense of a 'major redesign' of the primary survey: http://www.bls.gov/cps/cps_htgm.htm ... the Government conducts a monthly sample survey called the Current Population Survey (CPS) to measure the extent of unemployment in the country. The CPS has been conducted in the United States every month since 1940 when it began as a Work Projects Administration project. It has been expanded and modified several times since then. As explained later, the CPS estimates, beginning in 1994, reflect the results of a major redesign of the survey.