I think it's because out brains are running so fast we don't even realize that our mouths are moving. We are long past that thought, but our mouths just caught up, and our brains aren't thinking about it so out it comes lol I know for sure I am a rambler when I am manic. Not about anything in particular either, I just can't seem to shut off and a lot of the time I don't even realize I am doing it