Maz...
I think behind your head tends to create a bad angle for the shoulder girdle.
Let's just see, from Shoulder Impingement Syndrome By Dr. Clay Hyght:
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Exercise Selection
There are certain exercises that have a tendency to cause or aggravate shoulder impingement. For starters, unless you're dying to know what this injury feels like, never do behind-the-neck presses.
Next, any shoulder press where you elbows are straight out to the sides can aggravate the condition. A safer press can be done by keeping your elbows more to the front - as when doing Arnold Presses.
Most shoulder press machines give you two sets of handles to choose from. One set that runs parallel to your body and is wider, and another set that runs perpendicular to your body and is narrower.
For the sake of shoulder safety, use the narrow, perpendicular handles and keep your elbows in front of your body as opposed to beside it as with the wider handles.
Lastly, you may have to put your ego to rest and not perform standard bench presses. Although not benching is a sin in many weight-lifting circles, benches seem to be the culprit in many shoulder injuries.
Instead, try inclined BB presses, which seem to be less injurious to the shoulder complex. Besides, your upper pecs could probably use the work.
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