High iron content on the slab. It's rust. And it will continue to rust, especiallyif moistened.

Bill Vincent, our Kitchens Forum resident stone and tile expert (who hasn't been on in a while - we miss him), said in an old thread "cheap carrara is notorious for having iron deposits in it, and those iron deposits can and will begin to rust after a time, causing yellow or orange rust stains."

However, I do not know how you are supposed to know whether you have bought the cheap carrara or not. I know people have carrara marble in their kitchens for years, even decades, without rusting. Anyway, here is another thread where someone posted their solution.

Here is a link that might be useful: Older Thread about Carrara Marble and Rust

Drill 'em and fill 'em if they bother you that badly.