I am unsure where the rocks come from when farming. I used to think all the fence posts with welded wire columns filled with rocks were the coolest thing ever. I now know it is almost a necessity. Rock projects are a must. If you don't find homes for the rocks they end up back in your field. Where they bend the shit out of your equipment and break your shovels.
The pigs love to dig them up and throw them. I have seen a pig throw a rock she dug up about 10 feet across the yard with her snout.