What might be causing my shoulder blade to crack?

Joint movement noise. Cracking a joint moves the parts of the joint, with the cracking noise coming from cartilage surfaces sliding, from movement of a little air-space from one place in the joint to another, or from some other non-traumatic event. If there are no pains, stiffness, nor restricted movement before or after cracking noises occur, there is probably no damage being done. A doctor can examine it to be sure.
Snapping scapula. A crackling and popping sound that seems to emanate from the area of your scapula is likely due to a snapping scapula. The actual snapping and popping is from the underlying redundant bursa that crackles and pops as the scapula traverses across with protraction and retraction.