Often it's the result of not being able to establish the proper shoulder blade position due to tight pecs and upper traps.
You don't want to fully externally rotate overhead though. Don't go nuts.
Stretch pecs a ton, stretch upper traps, strengthen middle and lower traps, more presses, snatch presses, push presses and snatch push presses from behind the neck focusing on maintaining scapular retraction and not allowing significant elevation.
Also make sure your squat/trunk position is good. If you're leaning your chest forward too much, you'll be out of luck no matter how flexible you are in the shoulders.