I would take a guess that they use a front end loader to dump the wood into the kilns or conveyer belt.
Figure the wood is sittin in a yard that has gravel,rocks and such and I could see how some of that gets mixed into a bag.
Some of what I found on lump has been pcs of the kiln that has fell off (clay,mortar) which looks like you're pic.
You also have remember that when you cut a tree down it hits the ground hard, any rocks or stones will get imbedded into the trunk.
Tim