What is happening to American youth?

These numbers are for high schoolers:

From 1991 to 2015, seat belt use was about 3.3% higher each survey cycle compared with the previous survey cycle, adjusting for gender, race/ethnicity, and age. After 2015, seat belt use was about 1.8% lower each survey cycle than the previous survey cycle, adjusting for the same covariates.

Here is the article, and for the pointer I thank the excellent Kevin Lewis.