Is this the first time you have asked him to pull his head around to the side? and how far are you pulling/asking? does he need to twist his head in order to reach around back? I mean, so that the long line of the nose is no longer vertical, but at an angle?
He is likely stiff somewhere and if you are pulling too much, it causes him soreness. you might do thins little by little, giving nice full releases in between each one, and allowing him to walk on a loose rein as part of the whole excersize.
can you also do carrot stretches with him, on the ground, when you go get him and tack him up?