I just read a swim article stating: "Having lungs full of air causes legs to sink -- empty lungs to remain buoyant". Apparently air is heavier than water in certain areas of the world, possibly on the moon too.
Not having enough air in lungs causes hips to sink, this is just a matter of physics on earth. I often notice swimmers empty lungs too quickly in an effort to dump co2 or used air, but then drop 2" or more and then never find air due to lack of buoyancy. Slow continuous exhale is important, but the bulk of exhale happens when rolling to breathe.
Try the cannonball test. In the pool, fill lungs full of air, move into a cannonball shape with knees to chest, head in water, body stable on surface in rounded shape - begin to exhale quickly and discover how much air you need in lungs before dropping to the bottom of the pool. Every -body- is different, some will sink fast (mostly guys) and others drop slowly or remain at the surface. This test gives each swimmer an idea of how much air is needed in lungs to remain buoyant and balanced.
A good example of the body sinking without enough air in lungs: https://www.youtube.com/watch?v=c3VV4PwIBCQ