Kinda sounds like a similar problem I had. I was trying to find the video on crossfit where the guy talks about your shoulders being tight. His idea is that you picture a couple mechanical arms pulling a weight up and down with the same ball socket but they are essentially held together by rubber bands. As the weight goes up the bands stretch but then the weight is dropped overstretching the bands. This is what occurs with the ligaments and tendons for a lot of folks that are doing pullups. The analagy applies to all the joints. The problem most of us have is we don't really engage the muscle until we start pulling and then we disengage partly or in whole the muscle on the way down essentially stretching our "rubber bands" again. This may be your issue...
In his video the idea/theory is that if you tighten the muscle and engage them going up and down you reduce your risk of injury. This is also true for a lot of weight lifters because if they don't tighten everything up before the lift they just don't lift as much or eventually achieve injury. Pavolich at dragondoor.com aka "The Naked Warrior" states that keeping the muscles tight and strong before, during, and upon completion of whatever lift your doing is essential to the devlopment of staying injury free and getting stronger.
Take a look around on crossfit or look up mobility exercises crossfit on a search engine. There are a lot of videos and instructions that talk about keeping it tight as you go through your workout. Personally I think a lot of people are scratching their heads when a coach says keep your back tight but the main idea is your clenching your muscles up as you do the exercise and pavolich explains this pretty well. With my shoulder injury I've had to focus on doing that and it seems to be working.
"Permanence, perseverance and persistence in spite of all obstacles, discouragement, and impossibilities: It is this, that in all things distinguishes the strong soul from the weak”