Answer: Muscle being burned during exercise is determined by a couple of factors: 1) calorie intake (including carbohydrate (CHO) and fat intake) and 2) duration of activity. If total calorie intake is very low or if CHO intake is inadequate, then the body will have no choice but to liberate energy from muscle stores, effectively "burning muscle". This is why you may hear carbohydrates described as protein-sparing. If there is a sufficient amount of CHO coming in, then it can be used for energy, thereby "sparing" the protein for muscle building or other structural roles.
As duration of activity increases, energy stored in muscle and the liver (glycogen) diminishes. Amino acids from muscle (specifically the BCAA—branched chain amino acids) can contribute part of their structure to glucose production during prolonged bouts of activity. This generally does not occur to any great degree until well over an hour of continuous activity, but can contribute up to 10% of the energy used in very long endurance events.
Eat correctly and enough, and take your multivitamin/mineral every day and you will not have to worry about muscle loss. If your goal is to build muscle while losing fat, be sure that your calorie deficit is not too great (no more than 15% below maintenance) and enhance the recovery process and results by using immediate pre- & post-workout shakes/bars and/or branch-chained amino acids. Want more info? See Xtreme Muscle Stack: Creating the Perfect Anabolic Storm.