Well, there could have been other possibilities, like his chute failed to deploy correctly, or a possible suicide in the sport he loved...etc.
/Just sayin
I would say that the most logical explanation is that he expected the fog to act as some kind of reference to his altitude. Fog has a nasty habit of changing quickly and disorienting jumpers who pass though it. The biggest mistake was that neither Erich nor Lunder planned a timed delay, but instead were relying on Erich to reach for his pilot chute. That was Lunder's signal to pull. Any lower and we might have had two fatalities.
Lunder actually saw Erich hit the ground. He attempted CPR but realized that Erich's neck had broken after attempting to tilt back the head. I haven't asked much more of him since the memorial service.