Well to be honest, and no matter what anyone else says, groups of people leaving are the biggest cause of promotions within the community. This is true more for the higher ranks (XS, XD) than for the lower ranks (captain, general). This is because when a group of people like this leaves the clan, it creates a power vacuum that must be filled one way or another. Now for the higher ranks, it's often difficult to justify a new XD promotion if there already are enough XDs (though the same principle works for section leaders also). It's the same for general: if a clan isn't big enough to split, the only way that a captain will get general in that clan is if the current general leaves, steps down, or is demoted.