I am close enough to the edge of town that most any pleasant evening I can hear a lawn mower or two running. I have never heard one running after dark. The only person I know around here to hear of such a thing is a friend who had a tweeker for a neighbor. He woke up at 3:00 AM to the sound of a lawn mower running. Went out and looked and the tweeker had taped a flashlight to his push mower's handle bars and was mowing away.
Maybe it is a regional thing or the fact that it stays light here in the summer until 10:00. When it is light at 5:00 AM and stays light until after 10:00 PM why mow in the dark. But I have never heard of this before.