The whole point of a stoning as opposed to any other means of execution is that it can't really be said who threw the fatal stone, thus absolving the whole crowd of the sin and crime of murder, at least in theory. I got the impression that they were drawing it out on purpose to "teach a lesson" with the throws to the back. If they wanted it done faster they could bury them up to their neck to make for a smaller, more vulnerable target. Although to be fair, rocks are pretty heavy and they aren't the easiest thing to throw.