Are there certain scenarios that can lead to one to lose muscle over fat stores? I’m interested in how this can happen to people who workout regularly but I am still curious about all situations.
I would imagine even if working out, if you are eating sub-optimal amounts of protein and other nutrients muscles need it might compromise the rate of muscle growth and maintenance.
Lack of exercise will also cause muscles to shrink. So (SWAG) if you sit in your ass and carb load all day I imagine you’d generally be pre-disposed to pack on fat vs muscle.
Given the metabolic costs and benefits of muscle and fat, it’s more interesting to ask in what scenarios you can lose fat rather than muscle (or pack on muscle rather than fat).
I believe fasting does this 
Muscle is more costly for your body to upkeep - it takes more calories per ounce to maintain muscle than fat. Fat, once created, pretty much sits there until needed. Muscle needs all sorts of maintainence, repair and feeding.
So, if you don’t need the muscle for work or exercise, it’s in your body’s best interest to get rid of it and keep nice shelf-stable, cheap to maintain fat around instead.
Muscle - it really is “use it or lose it.”
I am interested in worse case scenarios. I am already familiar with how to lose fat, so I want this thread to focus on what can cause me to lose muscle yet retain my fat.
Don’t exercise, but maintain your present food intake. Without exercise, or even by reducing your exercise, you’ll lose muscle and maintain fat.