Why do horses "cock" a hind leg?

When a horse stands on three legs with one hind leg touching the ground only at the toe and that hip dropped we call it "cocking" or "resting" a hind leg. The latter describes the root cause better - horses generally do this when they're relaxed. They'll usually switch legs at intervals. Yes, they'll do this when you're riding if you're just chilling out waiting for something (it can be a bit disconcerting). However, if the horse has the hoof off the ground or is looking backwards, they're not relaxed at all - they're seriously considering kicking at something.

You can clearly see how the center horse has a leg cocked, exaggerated by the white sock. He probably just woke up when he realized the camera was pointed at him.