Judgment. The prefrontal cortex, often called the control center of the brain, is responsible for judgment, behavior and impulse control. It's one of the areas of the adolescent brain most affected by alcohol.
Drinking alcohol increases the risk of trauma in two ways: the likelihood of being injured and an increased seriousness of the injury. It stands to reason that if someone is intoxicated enough to slur their speech or not be able to walk straight, they are at greater risk of incurring an injury.