You are probably at least partyly on the right track with the blood pressure loss. Also the blood carries oxegen to the brain and that function can be interupted. The artical I read by Jim Carmicheal where they studied the effects of a bullet that stuck while the heart valve was open went into that kind of stuff. I can't recall how long ago I read that but they were talking about just that difference in reaction to bullet hits. Like why does one animal drop in its tracks with certain hits while the next animal hit in the same place may run off before it falls. This heart valve thing is what their studies came up with.

As also was stated here, spine and brain shots are usually instant death or at least put them down to stay. Nothing seems to do this faster than a hit above the front shoulder blade and right under the spine. A second shot may be required but it put them on the ground to stay. I use that shot at times if I feel that I must stop the animal in its tracks for any reason.

I would guess that by contacting Jim Carmicheal at Outdoor Life Magazine, a guy might be able to get a copy of that artical. Its interesting if a guy likes to know what makes things happen.