Tightness in the back of the shoulder is very common in CrossFit. Everything we do with our arms in CrossFit taxes those muscles in some way. If you don't keep up on it, the tightness will eventually effect your performance.
If you have difficulty getting rotation quickly over the bar with Muscle Ups, difficulty getting into your starting snatch or clean position, or unable to get your arms fully overhead with your head through, it could be tightness in those muscles.
This stretch addresses that.