People are going to read this article and deny that they are the culprits, but I know better
More times than not, when I am in the gym, I see people using the lat pull-down machine incorrectly.
Some people crank out their reps way too fast. First of all, under most circumstances, a proper repetition takes between five and seven seconds (but that's a whole different article). Going too fast on this exercise is very dangerous: the faster the weights travel, the less control you have. Your shoulders are jerked upward, putting too much strain on the supraspinatus (one of the four muscles that make up the rotator cuff -- and the most often injured).
The other common mistake I see is people bringing the bar behind their heads instead of in front and to their chests. This exercise is designed to focus on the latissimus dorsi (upper back). By not bringing the bar toward your chest, you are restricting the range of motion for the latissimus dorsi, thus not getting the full benefit of this exercise.
Another reason you should not go behind your back is that it makes this exercise too dangerous. It stresses the subsapularis (another muscle that makes up part of the rotator cuff, which stabilizes the shoulder and rotates the arm inward.)
So the next time I am in the gym, don't let me see anymore behind-the-head lat pull-downs.