One-car crashes tend to be less severe than multivehicle ones, hence
my wondering about seatbelts. With ice he may have been moving more
slowly than he usually did. Being old, it's all the more likely that
he was going fairly slowly. Head-injury from contact with the A or
B-pillar?

Rollover? Perhaps he slid down an embankment and hit a frozen rut or
something, enough to turn the van over.

It may be that, being old, he had a coronary. This certainly
happens... some of these drivers are dead before they crash... for
their sakes you'd hope so.

However those who don't wear belts and have accidents get what they're
asking for (I'm not saying that this guy wasn't wearing one, just
speculating). Relatives notwithstanding.