Context, context, context.
Yes, past perfect may be used to introduce "before-past" events; but the past Reference Time (RT) which such events precede does not have to be explicitly mentioned in the same sentence. Consider, just for instance, two distinct contexts in which this conclusion might be uttered:
The driver left Boston at about 10 am on the 22nd. The crash occurred at 1:40 pm on the 23rd, sixteen hundred miles away; he must have driven† through the night, without stopping to sleep. He probably crashed because he went to sleep while he was driving.
This paragraph puts the sentence in the context of a present judgment. It is uttered (Speech Time) in the present, and it narrates prior events in the simple past, locating those events at a past RT.
Sgt. Turnbull gave evidence for the Highway Patrol. He showed that the driver had been on the road for more than twenty-seven hours without sleeping. He had probably crashed because he had gone to sleep while he was driving.
This paragraph puts the sentence in the context of a past judgment, uttered at the time of the hearing, a past RT. The events it narrates occurred prior to that RT and are consequently expressed with the past perfect.
† Note that this is a modal past, not a perfect.