A decline of only 0.1 years doesn't sound like much, and indeed it isn't. But life expectancy in the U.S. has risen steadily and consistently for over 45 years, so 2015 represents a noticeable change. The only other year in those 45 years that life expectancy declined was 1993, at the height of the HIV/AIDS epidemic.
This time around, there doesn't seem to be just one obvious cause. In 2015, eight of the ten top causes of death (including suicides and accidents, as well as chronic diseases such as heart disease and diabetes) showed an increase in death rates. If life expectancy continues to decline in future years, health officials will have to dig deeper to try to discern the cause. For now, a wait-and-see approach seems most appropriate.