US life expectancy has not declined for two straight years since the early 1960s. Then it was two years of a very bad strain of flu.
Life expectancy is down for two reasons in the last two years:
1] Overdoses from heroin and opioids
2] Penicillin resistant infections
Heroin and other overdoses coupled with other forms of accidental death (mostly car crashes) are now the third leading cause of death behind heart disease and cancer.
See: https://www.nbcnews.com/storyline/americas-heroin-epidemic/u-s-life-expectancy-falls-second-straight-year-drug-overdoses-n831676