I think it's sort of a "bigger picture" situation, and not any one particular piece of the equation.
A friend of mine used to bulk up and eat as much as possible. He'd hit 215 with ease. Lift heavy, no cardio, and lots of protein. Then latter on he'd cut up, by eating the same protein, little to no carbs, doing 2hrs of cardio and lifting dinky ass weights that would make a fitness bunny laugh.
He would lose muscle because he wouldn't be using it. Even with consuming equal or even more protein, he'd lose muscle.
In comparison, I've never really lost muscle when dieting, for I feel several reasons. Sure I lose a bit of strength, but after you're doing an 8hr work day, an hour of weights, another hour of cardio and getting 6-7hrs of sleep/night, after a spell you will lose strength to some degree. But if you squat 315/8 and you're suddenly doing 225/4...there's something more to it than protein or carbs!
I think the key is to lift the same weights year round. There is no reason to lift lighter because you're cutting up. If you can squat 315/8 while bulking, then try it for 6-8 when cutting up. Don't squat 275/4 just because you want to get leaner. Constantly fight against what your body wants to do- homeostasis- and force your body to adapt while dieting to heavy weights.
Like the old saying goes "Use it or lose it." Another; "if you can lift 400lbs, why even attempt 200lbs after your warm up? Lift what you can."