I think that evidence disappears really quickly. I saw another video of a guy (the dark green shirt guy, like army shirt, with his sleeves rolled up, visible in the above video) leaping over barriers and RUNNING over to the large video camera on the tripod, and starting to pick it up. And that would have been the camera that caught everything, I think.

Or maybe the people standing around would have been blocking the angle that video camera had.

It did seem like they were specifically getting positioned to block the view of the phone/gun, and the only reason that whoever took the original video was able to capture it, was that he was higher up.