Someone running in your backyard may not have ill intent. He may have been being chased by someone(s) who did have ill intent. They could be a teenager just running around and knocking trash cans over and such. It could be someone who just escaped from being kidnapped. There are multiple possibilities.
Here in my state, there was a recent case where a woman entered a house late at night, and a man fired a shot at the door. The woman was struck and did live. The man was her grandfather, and she had been staying with him for awhile. He had fired in the dark.
Now, that said, I used to live a rural area and I recall one instance where the police where called about persons coming onto the property looking for someone who lived there, he wasn't there. It took the police 45 minutes to arrive. There is a reason everyone in that area kept a shotgun on deck.