I'm not sure you'll find many kids hitting the bar with their head back and an open shoulder angle. I'm sure most with a shoulder angle also throw the head. The "pullover" you describe is the angle between the body and arms I was speaking of. I may not have been clear. Sorry about that.
actually, that happens all the time. when it's wide open and head back they hit their head on the bar.
so i'll clarify a bit more what is happening with the muscles.
when you swing thru the bottom and the head and chin are down (NOT IN, have the kid hang in long hang and then gently press down on the top of their head so they know what down is) the body down swings to its lowest point towards the floor. it is THIS that is the most difficult part of the "tap". and then maintaining this position thru the bottom and the ensuing forward swing and the completion of the 2nd tap...whether that be for another giant or to release a flyaway.
because of vestibular reasons or just plain incorrect basic swing technique or because all that gravity at the bottom makes them "feel" (code word: vestibular and proprioception in the hands) they begin to "pull" on the bar thru the bottom. when you pull at the bottom you tighten the biceps, triceps, lats and pecs...the very muscles that should be elongated and relaxed thru the bottom. this then causes them to shorten their body (at the wrong time and angle) creating even more torque in to the hands and that "feeling" that is scary and something bad is going to happen.
so then, they then begin to rotate or throw their head backwards because they "think" it rotates them faster so that they can get around and not hit their head on the ground. when you combine the above paragraph and throwing the head is where you find the shoulder angle close up and the trajectory of the body heads towards the rail. the secondary problem, and usually after they hit the rail just once, they then "think" the correction is too let go at the bottom of the swing. and we all know that's wrong and what that looks like.
if coaches would take the time to talk to their charges about this they would hear their kids tell them precisely what i have just stated.