I finally put two and two together Thursday, and came to a realization of sorts, while running and shooting a couple more videos of myself like this one.
To begin with, I found that I'm still rotating my left arm around in front of my chest, in spite of focusing on changing it over the past couple of weeks.
So, I ran a bit while making a conscious effort to not let my left hand cross too far to the center of my body. I found this to be surprisingly difficult, and I had to push against some muscular resistance to get it back. Not a lot, but noticeable. I finally realized this is probably due to tightness in my shoulder, which I'd noticed already before. For example, I can't reach my left hand over my head and as far down my spine as with my right hand. By contrast, my right shoulder is very flexible. I think what's happening is my left arm does not swing as freely towards the back, and so it swings in a rotating motion around to the front of my chest instead!
Interestingly, when I managed to get my left elbow to travel far enough back, I found my balance was better and I really nailed the whole Mt. Galbraith trail, feeling totally in control without a single misstep. I ran a PR by 30 seconds, and it was accomplished with medium effort, and with stops for an unusual number of hikers.
So, I'm going to try loosening that shoulder up, because something is not right. I don't remember injuring it, but maybe I did a long time ago. Whatever the case, it just doesn't have the same range of movement as my right side, and it needs some work. An interesting problem to solve.