There is no reason for a horse to open it's mouth, unless he is being hurt by the bit.
Fighting the bit, head tossing, odd-angled head tipping by Icelandic Horses has become the norm.
It's the accepted "look" of tolt.
It does not seem that anyone recognizes it as a problem.
If there is a question about it, it is excused as "the horse wants to go", or "it's a show, they're all excited", or "he loves it", or "the other horses are pumping him up", or "it's the speed" (no matter that it happens on the trail, or walking, and / or when the horse is alone).
It is not recognized for what it is: a problem for the horse, a painful situation.
But we are making it better for the horse; and the horse is glad.