The reasons seem clear–it is summer, so school is out, teens are driving more frequently, and driving more often in unfamiliar areas. Additionally, teens are less frequently sole occupants of their vehicles–they are often driving with friends, perhaps a more significant source of distracted driving than cell phones. The National Safety Council estimates that passengers increase the risks of crashing by 44% or more.
For us non-teen drivers, it means we should be even more aware while on the road. Inexperienced drivers like teens, particularly distracted teens, have slower reaction times. Aggressive driving near those novices is more likely to result in a crash.
For parents of teens, we can take a page from the book of many states, which restrict passengers for newly-minted drivers, often prohibiting passengers in the first year or two of driving.