He was picking his brother up from a friend’s house. It wasn’t where he lived and he was unfamiliar with the area. If you search for the original news stories from when the event took place, more detail is given.
Maybe there was a typo in the address he was given. Maybe the house didn't have clearly marked address on the front. Or just maybe your question has nothing to do with a gun nut shooting anyone that knocks on his door.
Tbh it doesn't matter if he had any business at the house. He could go there to ding dong ditch, or ask a survey question, or complain about their yard, or ask how their day is going.
None of these things are a legal reason to shoot someone knocking at your door.