Passer du coq à l'âne. I love this expression, probably because I do it from time to time.
It's when you're talking about one subject and then you jump to a completely unrelated subject with no transition at all.
To be truthful, there is always a link for me, it just may not be apparent to everyone. And I am usually able to loop it back around so that, in the end, it all makes sense. I have a few friends who have similar conversation styles so they trust my sense of direction. Those who don't usually smile and watch me draw swirls.
Conversations are spirals and swirls, as opposed to straight lines, aren't they?