Mine does something like this, I call it chomping on the bit though because it's essentially what she is doing, but she lays on the reins and tries to pull them out of my hand while she's doing it. If I let go of her, her head will go straight to the ground, so sounds like yours may be doing something similar? With my mare, it's because she is stressed, I call it "brain fried", and it happens when I ask a little too much of her. Usually if she starts doing this, I'll back up a step and work on some basics that she knows really well until she chills out.
You say it occurs about an hour into your ride, how much contact is the horse on when you ride? Do you ride on a very loose rein, or with some contact?
"The times when you have seen only one set of footprints in the sand, is when I carried you..."