It seems to me that it just happens at complete random, but when they do the emotion that shows they have something they want to tell you, that's when you'd see it pop. I've gotten a few so far, but my best guess for farming new builds is to stay outside a long time and don't get close enough for villagers to see you and it should increase the chances of them having something important to say. There may be a more legit method, but that would probably do it over time.
Good luck on that.