Hollow Back   

A hollow back is likely derive from one of two widely different causes.

Firstly, there is the sway back, a conformation fault where there is a concave curvature of the spine between the withers and the loins. This is a congenital problem, often assosciated with long backed horses,and shouldn't be confused with the hollowing of the back seen in old age.

Normal spine of a level back	Concave spine of a hollow/sway back

From a rider's point of view a horse with a sway back may be easier to ride, because the movement of the back is flatter and quieter, making a more comfortable ride and it is easier for horse to change leads, but it is a weakness that can cause problems in the longer term.
The ability of horses and ponies to carry themselves and their riders effectively depends on their ability to elevate their back and loins, requiring strong back and abdominal muscles. A long back is flexible, but makes it harder for the horse to stiffen and straighten the spine to develop speed or coil/flex the loins to collect and engage the hindquarters to thrust their rear limbs forward. Reduced flexion also forces the horse to jump flatter with less bascule (curve in the spine).
It is difficult to develop a long back's muscle strength, so a horse is more likely to get muscle fatigue under the rider and to develop a sway over time. The abdominal muscles have more difficulty in compensating, so they are also less likely to develop. Loins and hindquarters may swing more than normal, increasing the occurrence of sore muscles which leads to a stiff, rigid ride. Cross-firing or speedy cutting likely at high-speeds from a horse with a long/sway back.

Secondly, a hollow back can be a means by which a horse or pony is try to avoid pain. Some of the more common causes of the pain that they might be trying to escape from are:
1) Poor riding habits.
2) Painful and bad fitting saddles.
3) Dental problems.
4) Too much weight for the size of the horse.
Most frequently it is the riders that are the cause of all four problems at different times.