From what I have seen on TV hunting,I would call that pure dumb luck. Dropping an animal in it's tracks usually comes from hitting shoulder blades and their explosion causes massive organ destruction. I.e. the shoulder drops the animal, but the done and bullet destroys the organs. I used to think thi was a great shot, but it seems like the organs being hit by bone fragments leaves alot to chance.

A low shot behind the shoulder should hit the heart and that is a sure things. It also gives a low exit hole for massive bleeding. This usually results in a step or 2 before they fall. High shots go into the lungs, which is deadly, butthey take longer to bleed out so they can run more.

Shoulder shots can drop immediately, but then get up and limp off to die slow.