Apparently the fog wasn't on their route, they were directed into the fog by ATC, a holding pattern.
Pilot had climbed to around 2000 ft and saw he just missed the top of a mountain / hill.
He turned and for some reason descended.
Fog can be very disorienting, even to the most experienced pilots. he may not have noticed he had dropped altitude.
From the most recent report it was CFIT, pending NTSB investigation. But some of what has come out has been audio and radar data from the local ATC.
