Had some one on here with a similar problem last week.

Probably an imbalance between your chest/anterior shoulder muscles (punching muscles so they tend to be over developed in boxers because obviously we spend a lot of time punching... ) and your rhomboids/rotator cuff muscles.

Any chance you could show us a picture of you from a side profile? you probably have Kyphosis/upper cross syndrome.

If your shoulders slump forwards rather than sticking backwards then you'll know that you need to strengthen your back muscles to get your shoulder into a better place.

Did this gradually develop or did you have some form of injury?

The clicking/popping is probably just the sound/feeling of the connective tissues clicking over each other/bone process.

Could be a number of things, I'd have to look at you really but there's a few ideas.