There are quite a few things that can cause white spots - but it's sadly common to see riding horses with white spots or marks where the saddle goes, normally on or on either side of the spine.
If a horse is injured, it is common for hair over the injury to grow back white (This is how freeze or cold branding, which I'll talk about in another post, works). White spots in the saddle area are generally a sign that at some point in that horse's life somebody used a saddle on it that didn't fit, resulting in "saddle sores" - bald patches worn by the saddle rubbing. Saddle sores and girth galls can also be caused by tack or equipment that hasn't been properly cleaned.
I've even seen one horse with saddle sores so bad the hair never grew back at all. Needless to say, he also had some issues about being tacked up.
Saddle sores are prevented by keeping equipment clean, always using tack that fits well and checking the horse for the beginning of a sore spot regularly (usually when grooming - this is one reason good horse people groom before and after riding).