Simple answer: Curiosity.
It depends on the composition. @Yisela had some great examples of focal point (and balance) here, I'm going to use one to explain my thoughts on the eye movement.
Obviously you focus on the the people in the center immediately. But take a second to notice where you naturally looked next.
For me, it was to the logo on the top left, then the Stokes logo and then the contents of latest news.
Coincidence, I think not! If you join up the yellow on this page starting from the most dominant focal point (the people) and working your way down in terms of focal dominance you'll see that it essentially makes a diagonal yellow line from the top left, to the bottom right and flicked up to the 'latest news' section.
Essentially the way I think about it is if I were to remove focal point A, what would focal point B be. This tends to be the way the eye travels. Also people are naturally nosey so we do tend to look at what we think others are looking at (in this case, logo top left).
I guess it could also be considered a reverse divine proportion. Given that you understand focal point already. Consider that a person looking at the focal point will very naturally go to whatever it is leading them to 'next' people are curious and impatient :) and once you've seen and absorbed a focal point, the element that is next is what then becomes the focal point for you as the original focal point is no longer the 'most important'. (In those seconds of course, this is most certainly not to say that overall the focal point is not important).
It is this approach that makes some surrealism extremely effective.
This piece by Vladimir Kush for example;
What a beautiful sunset, right? A perfect fit for everything that we've learned a focal point is. High contrast etc. everything about the image pulls our eyes right to that giant glorious, golden egg yolk.
See how there's scafolding supporting the eggshell - now isn't that a clever way to guide our eyes there later - post beautiful sunset focal point :)