I appreciate his theory, but I think he’s overlooking the fact that Hillary doesn’t seem to be able to see when she looks down (since her head injury and the prism glasses). Whether it’s double vision or no depth perception I don’t know, but clearly twice she could not tell she was stepping on a sloped step.
That doctor who was always nearby when she was campaigning was there to light up the floor immediately in front of her with a compact LED flashlight he always had in hand. It's quite clear in the video, and that ended the talk that he was holding a Diezapam (?) injector.
If you look closely at the video you'll see that each time she slipped she's stepped on the space between two stones in the stair. It looks to me like those areas are more slippery than the stones themselves.
That's not to say she doesn't have problems. A normal person would have learned after the first slip.