Maybe you're in the Twilight Zone and they were coming back to haunt you. As mentioned, you have to hit them in the head or squarely in the heart (tough to do).

On a side note, when I was younger, my Dad and I would hunt Rock Chucks across the Snake River canyon to zero in our deer rifles prior to hunting season (30-06 and 7x57). It they were on our side of the canyon, there wasn't anything left when we hit them.