Umm, what?
This guy, being a HCAC tech, carries huge objects in confined spaces. Being in Birmingham, he probably also didn't want to get made, because that would be the sort of thing that would cost him work. Combine all that with a possible tool belt, and apparently not a very well thought out rig, if any, and a Glock, and you're looking at a disaster waiting to happen. If I had to guess, one of his fingers probably found its way to the trigger while he was trying to hastily readjust.
One thing I've heard of is a finger being negligently left on the trigger upon reholstering. Push the gun back into the holster, trigger finger butts into the holster, pulling the trigger, and boom, negligent discharge. I suppose that might be possible too.
AIWB carry is pretty awesome when done right, because it can very well hide handguns. But it must be done only after the most careful consideration and selection of equipment. Otherwise this can happen.