Someone may be off topic and someone else may follow up, so they're going somewhere, just somewhere else. As far as the list itself wandering is concerned, I dunno.
Life is a slippery slope.
Every list though naturally can't turn into a community where you can live your entire life. No list offers more than one little slice of life. But given that we're lazy, it's so much easier going off topic on the list whose members we know than subscribing to another list dedicated to the other topic whose list members we may not know. Emotions blend, the intellect separates. I wonder if the on topic scopes on lists are all that different from such lines.