Anything that causes your body to eat protein more so than fat/carbs.
Being catabolic, overtraining, bad diet, lack of rest, low test, insufficient protein intake, high heart rate cardio (good in moderation, but higher heart rates signal your body to burn protein bc it's a faster source of fuel)
Lots of stuff can make you loose muscle mass. But if you're eating right, training right, and getting enough rest then you shouldn't have any trouble. Unless you're talking about loosing mass after a cycle.