Well, that's just it: there isn't one. An AD is just the term used to describe an ND. The distinctions I posted were my own.
There is a current (largely positive, IMO) trend to use the term ND, however the term AD can, and usually does, carry the same opprobrium. That said, it's mostly a semantic difference -- in some exceedingly rare cases, you might have an equipment failure of some sort, or get frapped by a bolt of lightning and be deemed non-negligent but anytime your pistola goes off and you don't mean it to, you'll have a whole lot of 'splainin to do!
In the present case, the only unusual element, unfortunately, is the part of the anatomy the poor guy put a hole through. The old "strap got caught in the trigger" is a time-honored way of blowing up legs and knees and toes. The traditional way of shooting your off hand is to try to hold the strap out of the way with our off hand whilst re-holstering with your gun hand finger is on the trigger.
Lou Awerbuck had a good piece about this in SWAT magazine a few months back -- worth looking up and reading. Lou's point was that it can happen to anyone -- all it takes is a fraction of a second's inattention, lapse, etc. and tragedy can strike.
There are only two reasons for it: Ignorance or Carelessness Whether you call it an AD or an ND ;
Thanks. Obviously, I see a big difference in the terms, and AD really annoys me.
I tend towards Glocks and Fobus Kydar paddle holsters with no straps. I don't have the retention issues to deal with, that LEO's do.