Thomson's book is Mahohboh, publ. by Safari Press.

I use a somewhat modified version of his explanation but that is another story.

In my limited experience, having an elephant fall and remain in the upright position occurred only when the elephant was standing still, on level ground, and was killed with the frontal brain shot.

I don't think you can count on it though, as others have fallen over with the same shot. Probably a fluke more than anything else, but I don't know. I have never read any discussion of it.