Your pec minor attaches to the scapula, and it can easily become involved as the shoulders begin to shrug upwards during a poorly executed pull-up. 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.

1. From a dead-hang, set your shoulders and lift your body up without bending your elbows.
2. After you've mastered that, do the same thing but allow your elbows to bend a little after the scap pull-up portion of the movement.
3. Progress to full-range pull-ups.