What you wrote makes perfect sense but then there was an entire sequence about how badass the Saviors are and how Rick's people are already dead even if they don't know it yet where the girl mentioned how the Saviors took every man and boy over 10, lined them up and shot them in the head. They could still load them in a dump truck, fill it with sand and dump the bodies but they wouldn't come back as walkers. Was that a major screwup by the script writers?
I don’t think the sand walkers were the ones who were killed by the Saviors. The women just fled their previous location. I think they (or someone else) dumped sand on dead people who were already walkers.