My boy has it. I have had people tell me that it is from how they "use" themselves, though im not sure if that is true. I dont think that it is that big of a deal. To most people it is just a confo. flaw.
Even some of the more severe cases 'usually' do not cause the horse discomfort with normal activity; certainly the horse might be limited in the riding area, which you would want to have a thorough vet work up to rule out any prolonged discomfort.
Some hunter's bumps are able to be almost eliminated with alot of back exercises, to help the horse develop stronger muscles over the topline.
My horse has this and it has not caused her any difficulty that I am aware of. She came off the trotting track over a year ago and I do believe that it is not as pronounced as it was when she arrived. Maybe from using different muscles now.
Its a pronounced bump on the hind end along the topline.
It can be more serious then some people realize. I had a gelding with a hunter bump, it turned out he had fractured hus pelvis at some point as a young horse and it didnt heal right. It can be conformational/ muscular or because of an injury or pinched nerve.
A horse at my old barn years back had a hunters bump from a girl who slammed down on his back every time she landed after a jump. He was terrible to ride. He'd throw his head, jump around, and cock his head awkwardly to the side in a way that made it really hard to do much of anything.
He's an extreme case-most of the ones I've seen are fine once you get the back and hind muscles strengthened.