thought of something today concerning rock throwing. Could it be possible that many of these instances are simply alarmed deer stomping their hooves. Whitetail deer will do this when spooked but cannot identify the animal that spooked them. They try to ID what spooked them by getting it (me) to move. As soon as they figure out your a person they're gone. Up close this "thump" sounds exactly like a rock hitting soil. This obviously doesn't account for people who see actual rocks.
I've been in the dark getting to a treestand and have had deer less then 20ft away blowing, snorting, wheezing, and stomping. I can see if someone wasn't familiar with these sounds and Sas on the brain getting pretty freaked out.