They'll pile up on a hill that isn't in a city radius (which isn't THAT stupid) but rather than immediately build a road so that subsequent pigpiling laborers can move in easier, THEY'LL JUST WALK TO AN EVEN FARTHER AWAY HILL! It's a good idea to activate a few when this starts happening to just build a road so that if the crazy AI decides to change its mind, workers won't be walking all over the place accomplishing NOTHING (which is another bug already many times mentioned in earlier posts).:crazyeyes