Am surveyor. Deal with fences over lines all the time. My assumption? It was either built while he was at work, so he just came home one day to it being there, or it's possible he had not ever actually had a survey before, so he had no proof where the line was and didn't want to start a spat over what would've been just his gut feeling.

Thus, he hired a surveyor after the fact who discovered the fence was over the property line and that his gut feeling was right all along. It's worth noting that most people aren't hostile, even when confronted with the fact that they built a fence wrong. But the ones who are unfriendly about it obviously are because they're generally unhinged anyway.

My response when a rare neighbor does disagree with my line markings is just "hire your own surveyor, but they're gonna find the exact same thing I did."

Turns out, they never want to pay $500-$800+ for their own survey, which is why their false interpretations about the lines exist in the first place.