Negligent homicide is the criminality committed when ones negligent actions are the direct and proximate cause of an others death. The negligent actions of the defendant must have directly caused the death of the victim.
In cases where the defendant was intoxicated or driving a motor vehicle under the influence of alcohol or drugs (even prescription drugs), the fact that the defendant chose to operate the vehicle in this condition is negligence on its face. If while intoxicated, the defendant causes an accident to happen, either directly or indirectly, and if someone dies as a result of that accident, the defendant is guilty of negligent homicide.
The theory is that if the defendant would have been sober, the incident would not have occured. Note that on many prescription drug labels, it says something to the effect of not operating a motor vehicle while taking the medication.
Additionally, the legal definition of the term includes the fact that it is a homicide that is committed "without malice" - meaning that the perpetrator's actions may have been negligent but they were not done intentionally with the desire to inflict harm.
Here is an example of one state statue that specifically mentions vehicular homicide:
"(a) (1) A person commits negligent homicide if he or she negligently causes the death of another person, not constituting murder or manslaughter, as a result of operating a vehicle, an aircraft, or a watercraft:
(A) While intoxicated; or
(B) If at that time there is an alcohol concentration of eight-hundredths (0.08) or more in the person’s breath or blood…as determined by a chemical test of the person’s blood, urine, breath, or other bodily substance.
(2) A person who violates subdivision (a) (1) of this section is guilty of a Class C felony.
(b) (1) A person who commits negligent homicide if he or she negligently causes the death of another person. A person who violates subdivision (b) (1) of this section is guilty of a Class A misdemeanor."