I second or third or whatever it is by now the advice to keep the tree in vision as you clear out along your escape path. In 30 yr., once a tree almost got me good because I ran away with my back to it. As the tree fell, it rotated, rapidly, and brought around a large limb I thought was no problem due to where it was located on the trunk. By large I mean about the thickness of my thigh, large enough than when it caught me and threw me down then pinned me, it hurt. Fortunately, it was just hurt and not damage. Startled the heck out of me, I can say.
Afterward, when I managed to dig beneath my leg to be able to pull out from under it, I thought about what had happened. Best I can figure, the various limbs on the tree had grown so that the tree, though standing vertical, was unbalanced and thus the rotation on its way down. It rotated about the vertical axis and naturally brought the limb in question whirling around. It was extraordinarily quick. Since then, there've been a few other times a tree did something sort of like this, though not with a limb in position to come around and hit me. In any case, I was expecting it and was in good shape to avoid getting hit.
Live and learn.