ADD. What you're talking about reminds you of something else, interesting or cool, which reminds you of another thing, so next thing you know you're talking about that without remembering the entire segue to the new topic happened entirely in your head.
When I'm driving in the car on long roadtrips and my daugher is sitting there, just looking at me it means I need to backtrack and explain how I got from what we were talking about to what I'm babbling about.