Well I think the body does it by a sneaky way - when you are low on calories, it switches on depression.
I think depression (not all its forms) is a survival tactic, the aim of which is to minimise enthusiasm for effort, shut down libido, shut down thinking (your brain uses 1/3rd of resting energy, our big human brains are the SUVs of the animal world) ...
If you starve yourself of calories you are still eating far, far more than the average, extremely hard working person in Africa who walks miles a day up hills doing the farmers walk with kilos of water ... and they can still be damned strong, lean and mean.
If however you cut calories and sit around all day, and don't have the energy to exercise, and go half assed with your intensity, well I think your body starts stealing away your muscles.
If you don't use them you lose them.
You body doesn't want to carry more muscles than it needs and will shred them first chance it gets.
If you cut calories you switch on depression (even in a mild form) and your body THINKS that there is not enough food - wipes out any muscle you don't need, saves as much fat as possible, shuts down your brain as much as possible, prepares for what it expects is going to be a horrendous, long haul famine.
The only way around it is to use willpower, exercise, make your body think it needs the muscles.
Personally I am not a big fan of reducing calories. Instead of cutting calories and doing aerobics for an hour, increase calories and do aerobics for SIXTEEN HOURS A DAY. Go take a few days off and do some serious hiking, with a heavy pack. (seriously you can do 8 hours a day not 16).