Let's Get Technical
I have found it to be relatively common that a horse has a tendency to drop their shoulder in a turn. It really is not so much a dropping of the shoulder as it is an off balance turn that falls to the inside. Typically when this happens I have seen riders try to correct it by lifting the shoulder. This may work to some degree but barely. It is not the falling shoulder that is the issue here. The problem lies in engagement.
A horse that travels in a unbalanced state and mostly uses their front end, is one that will feel as if they are dropping their shoulder. By working on shifting the horses weight to the hind end this trouble can be remedied. Though not an overnight fix to the problem it is one that eliminates a "dropped shoulder." By shifting the horses weight to the hind we are working on engagement from behind. Horses that were hurried in their training never learned to push from behind nearly enough. A spur to the shoulder will result in a quick turn but one that is unbalanced.
By shifting our focus to the hind end, we teach the horse to balance and turn equally with the front as well as the hind. It may seem a little counter intuitive that the trouble comes from the hind and not the front. The thing is, the front end is turning very well. The horse has learned to turn the front end quickly while setting the hind and more or less pivoting. When we can teach the horse to use the hind end as well as they use the front end, we bring more power to the turn. This, and there becomes way less of a chance that a horse will fall down in the turn.
Teaching engagement from the hind end comes from a variety of exercises. The main one that I use is riding in a small circle. Riding in a small circle with some life I can feel where the front end is trying very hard to turn while the hind end steps short and quick. The trick here is to not shut the front end down. The front is turning nicely. What needs to be addressed is catching up the hind end. By dropping my inside leg back and encouraging the hind end to work harder, I can create engagement. While in the turn, with my inside leg encouraging the horse with a slight bump, bump, bump, I can feel when the horse steps forward and through with the hind. It is at this moment that I quit the circle and allow my horse to walk off straight.
By walking off straight and relaxed we can teach the horse that it is rewarding to engage from behind. I travel a little ways on a straight line before I enter another turn. My next turn usually being in the opposite direction. This to work on balance. If at some point I feel one end working better than the other, I will spend more time turning in one direction than the other. When that side begins to out-perform the other, my focus goes once again to the weaker side. By spending some time working at this exercise I can begin to re-balance the horse.
Working at teaching a horse to turn equally with all four quarters brings balance to many movements. I say "four quarters" meaning front left, front right, hind left, and hind right. During a turn when we can feel each quarter exerting equal effort the horse is turning in balance. Spending some time at this will really pay off in several aspects of riding. By setting up this circle exercise, and feeling balance before going on to other things, one can advance a horse rather quickly. Try it and realize the benefits for yourself. I know it is a near starting point for me on any horse.
Thanks for reading. I hope this helps you.
Eric