Low carb diets are often used for fat loss, but it is a mistake to cut your carbs too drastically. Carbohydrates are protein-sparing, so even if you are eating large amounts of protein, you can still lose muscle if your carbs are too low. You are not eating enough calories to support muscle growth. This is actually the most probable cause of muscle loss. Your metabolism slows down and your body actually burns muscle tissue to conserve energy.
Muscle is metabolically active tissue, requiring a great deal of caloric energy just to maintain it.
That's why your body will shed muscle if it thinks you are starving. You are not training with weights. The weight training helps keep you from losing muscle while you are dieting. You are much more likely to lose muscle from not eating enough than you are from doing too much cardio. All too often, people are afraid to eat a lot and do a lot of cardio at the same time.