Changes in earnings and income are characteristic of a dynamic labor market, as people change jobs or careers, move between part-time and full-time work, or start or stop working.
Having that flexibility in the labor market is generally considered a source of strength of the U.S. economy overall. Nonetheless, that variability causes true economic hardship for some people.
This Congressional Budget Office (CBO) paper uses administrative and survey data to examine year-to-year changes in individual earnings and household income since 1984. It also examines variability in earnings and income by factors such as age, sex, and education.