In the link I posted, Dr. Deb gives information on how to fix it:
"‘Wringing the hocks’ is another term for it. The cause is rooted in the fact that the horse is not using his lower back properly. He does not know how, when moving under a rider, to ‘round up’ into a posture that will allow him to carry the weight and still be able to use his hind limbs normally.
"The cure for it is to learn how to use the same exercises I speak of here all the time. The horse must learn how to back one step at a time, how to rate before a grid of ground poles and then how to negotiate the poles with rhythm and bounce. He must learn to untrack, to leg-yield, and then he must become proficient at shoulder-in.
"None of this has even one iota to do with any form of competition. None of it is ‘advanced’ work. Rather, all of it is fundamental, the basic stuff that every horse must know in order to do his most basic job, which is, to carry a rider on his back.
“It will also be beneficial in the short term for you to learn how to perform a groin release. Horses that wring their hocks have excess muscle tonus or ‘tension’ in two areas: the superficial gluteal muscle on the front-top area of the croup, and in the adductor musculature that is on the medial side of the hind limb, i.e. the muscles that are between the hind legs. Use the Google advanced search function to look up Pauline Moore’s thread that explains in detail how to correctly perform limb stretching, and then also find the explanation I gave somewhere in another thread on how to do the groin release.”