I am quite aware of the pain you are referring to in the MU. I believe the same type of discomfort can be reproduced by that "arm up, bend the elbow behind the head" bodybuilding exercise with any heavy weight, but it seems to me that's probably not what you're dealing with here.
One thing that comes to mind is elbow overextension. There's also this one wrong thing you can do which involves kind of flipping your shoulders back about as far as they'll go and trying to get your elbows directly up towards the ceiling, which might involve making your torso not inclined forward through the bar as it should be... maybe torso inclination is the main thing with that one... okay, Greg can probably explain the error a lot better than I can, and I'm not even sure if that could cause this problem.
Steven Low will probably have something to say on this.