You didn't say the age/training of the horse, and the picture doesn't help. Two possible things going on.
(1) this horse has had too many novice riders hanging on his mouth
(2) this horse is green and hasn't been taught impulsion and doesn't listen to cues.
For the former, you need to retrain the rider.
For the latter, you need lots of lunging and in-hand and/or ground training.
Sometimes they're simply just stretching out their back and wither....my guy does this without the gaping....the more rein I let out the lower he goes. Not sure what the gaping is about though.....hmmmm?
Sometimes called pig rooting or bit evasion. After 15 years my horse started doing this. Even in a halter. He hated ring work so this was how he dealt with it. Never did it on the trails whether he was in a curb, snaffle or hack.
I have a horse in my program that does this same thing in the ring at the trot. I agree with Corporal the horse I have does this because of beginner riders.
Other then trotting what are you doing before it happens?
What do you do when he does it?
What is you riding experience?
Do you feel that you can fix this? If not do you have the option of talking to a trainer that you know and knows your horse?
The horse doesn't belong to me but to the owner of my facility. I have been teaching riding lessons for 5 years, I feel I am fairly knowledgeable and experienced for my short time teaching. I am also paid to exercise several horses. I feel I have more learning to do and experience to gain before I would feel comfortable calling myself a trainer, but I also feel that many self-proclaimed 'trainers' in my area need to do some more learning as well. I hope that gives an accurate and humble picture of my experience. The owner of the facility offered to let me use his horses for lessons. He hardly has time to ride. 3 of his 5 horses haven't been ridden in 5 years. His daughter has had a pony for 4 years and rode it once before she began taking lessons with me a month ago. He says this horse has had a lot of training and used to be 'like, a show horse in beauty contests', I asked if he meant 'halter' and he said 'yeah, that must have been it'. So, I started riding his horses to test them all out, this one has obviously had some good training. She moves off your legs well, respectful and willing, but has had a good chunk of time off. So I rode her and tuned her up a little before putting one of my most experienced students on her. She does this with me and with the one student I have had her on. With me, she seems to settle and quit doing it after a while working.
She is only 8, but has supposedly had 'tons of work done'.
To be very honest, I haven't experienced a horse doing this. So, I just continued riding and keeping her busy. I can definitely see it being caused by novice riders hanging on her mouth.
I do feel I can fix it, I'm just not exactly sure how to go about it. Thanks for the help. :)