Also don't underestimate how quickly panic can set in when you find yourself alone in the dark with no source of light.
I know one time that I was staying at LeConte Lodge and went to Cliff Tops for sunset. Because I had a flashlight, I stayed at Cliff Tops to watch the stars come out. My flashlight was small, and could only light the path for 10' or 15' head... and it turned out that wasn't enough to find my way back to the Lodge. On the top of LeConte, the trail is a pile of rock. But so is every drainage and water bar. My light was not strong enough to show me which was the path and which were just random rock drainage's. I started to panic because I wasn't sure how I was going to get back to the lodge. Fortunately, when I turned my flashlight off, the moon light was strong enough I could see it's reflection off the rocks. With that light, I could determine which set of rocks continued into the distance so that I could determine which way the trail went.
Now I can only speculate WHY Susan went off the trail above Huggins Creek drainage. But I could easily appreciate how she could start to panic, and when panic sets in, you don't always make good decisions.
Did she think the parking lot was somewhere below her and thought it was a short cut?
Did she first try to shelter in place and there was a wind that was chilling her so she tried to get off the ridge to get out of the wind?
Did she step off trail for any number of reasons (potty break) and get turned around and couldn't find the trail in the dark/twilight?
No way to tell. The only thing we do know is she stepped off the trail that evening or else the SARs that searched the trails into the night would have found her.