Yes, I've seen this in my long-time enduarnce horse (2900+ miles.) I just
let him stretch & keep going - never quite figured it out, although I see
it more when he's getting tired. If your horse goes with a high head, it
may indeed be back muscle stiffness. I also see it in my horse (who does
not normally carry his head excessively high) after he's been fighting me
at the start of a ride, wanting to go faster when I won't let him. So I
think it is tension-related. I'll be interested to see what others think.
Patty