This novel is committed to the idea of fate. The way the writing weaves back and forth in time and events are constantly foreshadowed reinforces the idea that everything that happens is meant to happen.
This is a pretty cool way of avoiding the feeling that the author is forcing the reader's hand. Think about it: usually when you meet a new character in a book, that person is going to become an important part of the plot. In this novel, however, the author's hand is superseded by the much heavier hand of fate, which creates a nifty little blind for the author to hide behind.
The indelible Ash Wednesday markings on the foreheads of the 17 Aurelianos that Colonel Aureliano Buendía fathers during the wars are part of the way this insistence on fate works. If you've never seen an Ash Wednesday mark, it's basically a small cross of soot in the middle of the forehead. But you know else these crosses immediately start to look like as soon as we realize they aren't washable? The cross-hairs of a gun sight or a target. These boys are literally walking around with targets on their heads, and each of them is gunned down by a bullet shot into exactly that spot.