Distractions is my bet.
...
Why bet when the article tells us that it’s because more miles are being driven?
Interestingly, some places saw higher fatality rates with LESS traffic a few years ago. Traffic fatality rates in New York City, for example, are generally very low because the city is so congested and it's almost impossible to lose your life in a crash during most of the day even if you're trying to kill yourself. NYC saw a spike in fatality rates a few years ago when traffic volumes declined.