I've never tried, but I assume maybe it's just because of an expectation of how you imagine a dream would react when trying to increase lucidity. Maybe unconsciously you just think that there should be some kind of reaction that can show you that the dream is becoming more vivid, and it manifests itself that way. And if it continues to do that it's probably just because of you seeing it the first time and having that expectation every other time. It sounds cool though and I hope when I try to do something like that it has that kind of effect!