Too heavy of a rider
Bad conformation

Bad riding (rough sitting or posting)
stiff muscles from too much work
vertabrae being out of alignment. (solved by equine chiropractor)
bad conformation, like a roach back or sway back.

Too much riding

Proper fitting tack that is crooked, or needs a new saddle pad, flocking etc.

Riding off balance

Hoof, hock, stifle or other joint problems. Just like with people, problems in the feet, ankles, knees can cause problems with our spines, so the same can happen with horses.

If you have access to an equine chiropractor or other therapist maybe get a check and see if they can help you pinpoint a problem.

Good luck, and hope your horse is better soon.

pets question and answers,www.5d2d.com

Lisa

http://www.tender-loving-horse-care.com

Quite a number of conditions could produce back soreness. Anything that produces inflammation of the articular surfaces of the joints of the vertebrae, or periarticular inflammations affecting the spinal nerve roots, ligaments, tendons, or muscles along the spine can be the cause. Possible causes of inflammatory changes can include infections, injuries, conformation issues, genetic abnormalities, nutritional deficiencies or developmental malformations associated with dysplasias of bone and cartilage, such as bone cysts or degenerative diseases. Working horses incorrectly can also lead to weakness and fatigue causing back soreness. Even slight stretching of ligaments that support the spine of a young horse being ridden before the vertebral growth plates are ossified can permanently loosen the attachments and lead to inflammation from instability of the intervertebral joints.

Registered Nurse and 57 years with horses

Too much riding

pets question and answers,www.5d2d.com

A rider flopping on his back
Bad conformation
Crooked tack
A heavy rider
A lumpy saddle
Joint or spine issues

May be you should check the corrals for rocks, I know horses are carefull about where they roll but if there are rocks where they roll he could have rolled over one and hurt his back, spine or his loin.

A lot of the horses I've seen with chronic sore backs have had hock problems. And almost every single horse I've known with hock problems has had a sore back. So I'd definitely look into that possibility.

Compensating from any type of lameness can make the back sore.

Bad riding. Not just bumping and thumping around on the horse's back either. If you let your horse go around with his head up in the air and his back hollow, his back can get sore. If that's the case, you can fix the sore back by teaching him to stretch down and round over his back so that he stretches over his topline.

pets question and answers,www.5d2d.com

Ciropractic problems - more common than you think!

Gastric ulcers. A friend of mine was spending a fortune on chiropractic, massage, acupuncture, etc to help with her horse's tight back, but when she treated her for ulcers the muscles of the back softened and the horse was more comfortable under saddle.

Previous injury.

Spinal malformation.

Mounting from the ground, especially if you are heavy and/or really pull the saddle sideways when you mount. Use a mounting block instead.