Toward one end? Didn't notice that in the original post. That's rather
perplexing. Weighting could still be the problem, though it would have
to be pretty drastic to have an effect. Double check the floor for
level. Check if the legs are plumb with respect to the ground on all
axes (Unless the are angled out for stability, in which case walking
would be really wierd).

The most likely culprit is human interaction. Rock it from all angles
and see if it continues its walk. You may be pushing or pulling it
slightly with each push. If that's the case, it may not be rocking
smoothly enough, so you have to compensate with a firmer push. If it
isn't rocking smoothly enough, that could be because of the floor (Are
you on a carpet?) or because the rockers aren't smoothly curved. Did
you cut them both at the same time or use a pattern bit in the router
to ensure that they are the same shaped. Even if you did, you may have
reversed them so they aren't rocking evenly.

If you figure it out, even if it isn't wood related, please let us know.