Why do my shoulders crack when I roll them back for warm-up exercises?

As long as there is no pain associated with the cracking noises, you really don't need to worry. Occasionally our joints just crack for no obvious reason (although soft tissue in the joint is generally the culprit). In fact most people experience cracking or popping of some joint, as they age. However, if there is pain associated with the cracking, that's a different story. Any popping or cracking or any of the joints that is accompanied by pain is probably indicative of damage to the surface of the shoulder joint. It's possible that you may have a labrum tear. Or, you may have what is known as shoulder instability, which means that your shoulder is slipping at least partially out of the joint. There could be other causes as well. But in any case, you should have it examined by a doctor. The pain associated with the cracking is not likely to get better over time on its own, and it's always best to begin treating an injury as soon as possible to prevent further damage.