This is what a small section of the back looks like:
When the horse's back is ventroflexed, the edges of the vertebrae can touch and scrape against one another. This may cause arthritis or be painful to the horse.
The Ring of Muscles is not working the way it should when a back is ventroflexed. Rather than the ring muscles contracting to hold up the back and the back muscles lengthening / stretching, the back muscles are contracting and the opposing muscles are lengthening.
Horses that travel in a hollow position a majority of the time, aren't developing strong topline muscles or loins, a good reason to switch gaits often, and work on rounding the back at the walk, trot, and canter.