Why would your neck pop right in the center of your spine where it joins your head?

Those are the bursa (the sacs of liquid between your vertabrae) suddenly expanding. It's harmless, really. It also means you have nice loose neck muscles. If your neck continually pops around the top of your head, I would not commend you on nice, loose neck muscles. A "pop" sound is not comming from a bursa between your vertabrae. Vertabrae, in fact, do not house bursae in any intervertebral manor. A vertebral disc is what sits in between two vertebrae (this creates a joint.) A bursa is a liquid sac that helps ligamentous and tendonous structures slide more easily past each other. These are found in larger joints, such as the knee, hip and shoulder. An articulation or a joint "pops". This sound comes from a release of nitrogen gas from the tissues that surround a joint, when there is a sudden negative pressure within the joint. This is harmless if it happens once in a blue moon, but if you experience this with any frequency, it is a sign of hypermobility or too much joint motion. This can lead to a problematic instability and even spinal degeneration. A chiropractor is the expert to see in this situation.