The youngster was all of 6-years-of age and out playing in yard on a trampoline when he was involved in a car accident. The car's driver had according to the KCCI news report rear-ended another car, then lost control and headed into this family's yard where the youngster was at play.
The man was charged, according the news article. I'm sure tests were likely performed by law enforcement to determine the driver’s sobriety. Pending those results it is difficult to say why this could have happened.
“Guadalupe Villa-Guzman, 41, faces multiple charges, including vehicular homicide, in the death of Irving Aguilar-Gonzales last month.”, KCCI-8 News