Well you are not directly above the people, but rather slightly off to the side. Basically the angle of the camera, compared to the ground is unnerving me slightly.
Honestly this comes from my fear of heights, and looking at this image actually gets me feeling as if I'm leaning over a handrail to take this picture (which I've done, so that kind of makes it worse xD)
Without checking the exif you used your stock 18mm zoomed out correct? The barrel distortion on those is so rough >_< I'm still stuck with mine. You did a nice job straightening the lines, but that wasn't what was bothering me as previously stated :x just the fear of falling haha