He states that the gun is unloaded and then walks off camera to have someone off camera verify that its unloaded. It sounds as though the gun's slide is locked back while off camera.
He walks back on camera and holds up the gun with the slide locked back.
He holds the gun up and identifies it as a "Glock forty".
He drops the slide and utters the unfortunate phrase that he's the only one in the room professional enough to handle the gun.
He then points the gun downward, pulls the trigger and the gun discharges.
Video quality is poor, but it's obvious from the subsequent chain of events that when he walks back on camera there is a magazine already in the gun with at least one round loaded.
That means that there are two possibilities.
1. Both he and the off-camera person who verified that the gun was unloaded failed to note that the magazine was in place and still had at least one round in it. This would mean that neither person knew how to properly clear an automatic or that both of them were quite careless.
2. The magazine was reinserted off camera by Paige after both people had verified that the gun was cleared. This would indicate two things. First of all that the person off camera was not observant enough to note that the magazine had been put back in place--he should have stopped Paige immediately. Second, that Paige was not being careful.
The legal challenge was ridiculous, in my opinion. If you do a public demonstration it's completely unrealistic to expect that it can somehow magically become private later on.