I agree, I don't believe they can determine what caused him to fall backwards off a cliff, only that he somehow fell. Why would he be standing with his back facing the edge of a cliff? Because someone was in front of him? Otherwise I couldn't begin to guess.
In my area in the mountains, we had (another) hiker fall off a cliff last week, I'm not sure why, but he was on a hike where people go to do technical climbing with all the gear, eg ropes, pick axes, special footware and many hours of training, but he wasn't a climber himself.
Without knowing the topography, it's not possible to speculate what happened to Joe. But first off, he was wearing running shoes, which are slippery on rock, and secondly he was in unfamiliar terrain beyond his experience and skill level, and was alone, so no one could help him. Who on earth would want to push him over a cliff? I think people need to get out of cities, they'll find that in the wilderness people naturally help each other if they can.
Sometimes someone slips and grabs onto something as they fall and then tries to climb back up but doesn't make it. Or, being in a rush to get back, they think they can climb down using natural ledges. It looks easy from above, but the distances are distorted.
IMO the dangerous, and in this case fatal elements are gravity, rock, inexperience, over-confidence, adrenelin and solitude.