A Horse carrying their head behind the vertical is a major fault in dressage. When a horse head is behind the vertical not only is the horse more tense throughout the body, but the horse is in a position where it is more difficult to control. For these reasons, a horse who carries their head behind the vertical must be corrected in order to successfully compete in dressage.
Many young dressage horses experiment with holding their head behind the vertical to attempt to evade contact with the bit. If your horse attempts to evade by getting behind the vertical, correct them by giving a bit of slack and applying leg and seat aids to drive them forward to fill up that rein.
Many horses develop the habit of holding their head behind the vertical to protect their mouth from a rider who is trying to pull their head back into a headset instead of emphasizing riding the horse forward into the bridle. Be conscious of riding your horse forward rather than trying to pull his nose back- as this action will create the habit of carrying a head behind the vertical.
Once learned, this is an effective evasion tactic for many horses and because it severely limits the amount of control a rider has, this is a problem that, once trained, may need to be fixed by a professional trainer or dressage rider.