I spent a week burl hunting with my uncle in Alaska. Good times! (except for the mosquitos)
He told me to do it for extra money, so one summer I went off on my own to start cutting them.
And that's the story of how I learned that sometimes burls form when a tree grows around a fence post. Alternate title: Stihl chainsaws can't cut steel fencing.
Most of them are old fence lines where someone decided attaching barbed wife directly to gum and hickory trees was a good idea.