Strange stick figures

I took a drive out to Campo, CA, which is about an hour east of San Diego and continued driving past it further out to the unincorporated area of Boulder Park. I was searching for abandoned buildings, and went down a no outlet road then saw something odd off the side of the road which you really had to be looking for to see. These stick figures were blocking tunnels below the road... I don't get what they were doing there for, but thought it was both strange and a little frightening considering no one was really out there.