I saw clinician Julie Goodnight address this with a horse. The term she used for the behavior the horse presented was "stargazing," which is when a horse holds its neck and head abnormally high. She said it could be caused by training issues, rough hands, severe bits, mouth or saddle pain, hereditary, or a health issue.
What Julie Goodnight did with this horse in about 20 minutes was amazing. She kept constant contact with the horses mouth when it was stargazing, then released all pressure when the horse lowered its head to escape the contact. At the end of the session, this horse was walking and trotting on a loose rein, head down, well on its way to correct the neck muscling.
There isn't a quick fix for this issue...it takes time for muscles to grow and develop and to break out of this habit. For some horses though, it is just how they are built, and in that case there may not be much that you could do.