Pull to the Nose?
The beginning movement for any pull-up or pulldown is to depress the shoulders to properly engage the back muscles. However, pulling ALL the way through to the top of the lift can demand a lot from the stabilizing muscles like the rhomboids and low traps as they struggle to maintain their strength of contraction as the lift progresses.
The thing is, you don't always need to pull your chest all the way to the bar during pull-ups to fully engage the back muscles. This is especially true if you've got long arms. Stopping a few inches short so the bar reaches nose level (as an example) keeps the back muscles engaged.
Your Scapular Depression Game is Weak
Your pec minor attaches to the scapula, and it can easily become involved as the shoulders begin to shrug upwards during a poorly executed rep. If you have this problem it's usually because of poor scapular depression skills. Using scapular activations (scap pull-ups) as their own exercise can help you properly learn to initiate the movement using the right muscles.
Reset Between Reps
But things don't just end there. A well-intentioned lifter may think he's staying two steps ahead of the game by not allowing the shoulder blades to rise up by keeping them packed down for the entire duration of a set of pull-ups or pulldowns. This will fatigue the muscles holding the shoulder blades down and they'll eventually tire out, especially with a heavy load. What you need to do is reset them between each rep.
How to Set the Shoulders
Scapular mobility is an understated skill that needs to be developed just as much as developing their stability. It comes from practicing the right techniques under low loads and making smart progressions.