When your elbow pops it could be the joint capsule being rapidly stretched. It releases gas which is the popping sound. In an unhealthy joint where the cartilage has deterioration, the joint pops due to friction. Healthy joints can pop from friction too.
As the elbow is a joint, it is probably popping in order to release excess air in the joint, such as when you pop your knuckles or back. If the elbow is popping out of joint, you should see a doctor immediately.