Muscle doesn't turn into fat, anymore than arms become legs. Muscle can lose it's tone and sag, and it can be covered in fat... but the muscle is still there, unless it atrophies and is broken down by the body. That's why it doesn't take nearly as long to restore strength as it did to develop it in the first place.Muscle is denser, and burns more calories than fat - but muscles that are not constantly in use quickly convert to fat. The food necessary to maintain such muscles during the training season is more than is required to maintain body weight - to maintain that type of muscle, the same level of muscle-building activity must be performed regularly; any drop in activity leads to a corresponding drop in muscle mass, and the changes are additive - the more muscle turns to fat, the fewer calories are burned, the more muscle turns to fat, etc. Only by resuming the previous level of activity (and the ingestion of any supplements, legal or not) can the person rebuild the lost musculature.
Bodybuilders weight is so dramatically different during their off season versus competition weights because they focus on reducing the fat in their bodies down as low as possible -- often to unhealthy levels. Add techniques to drive water out to highlight their muscle development, and you get some serious weight differences. Fighters and wrestlers also often show a significant difference in weight as they prepare for competition; many fighters carry something like 10 to 30 pounds when they aren't preparing for a fight. Ideally, they cut just enough weight to slip under the weight limit at weigh ins, and then go up 5 to 10 pounds for the fight. They want to really be a 180 pounder fighting in the 170 pound class, for example, to get the maximum advantage of weight and strength.