I HAVE had back pain due to the way a saddle sits me on the horse.
It first started when I bought a flex-tree saddle. Loved the saddle, but it killed my back. It got worse and worse and worse until finally I couldn't ride in it anymore and I had to get shots in my back. (I did not want to give up on that saddle).
Ever since then I am really susceptable to posture induced back pain while riding. If a saddle is a tad too wide for the horse or otherwise forces me to lean back, even minutely, then my back will flare up again. Instead of fighting it, I now realize that if a saddle hurts my back it's better to just sell the saddle then try to make it work. Luckily I have a great saddle right now that never hurts my back. But man, some saddles will just not work for me anymore. It seems to me that if the saddle slopes even slightly downhill on the horse, forcing me to compensate by leaning back ever so slightly, it is the kiss of death for my back.
So yes, I believe a saddle can cause back pain. It may not be the saddle's fault, but if you already have back problems then the posture of the saddle can cause pain, at least in my case.
I ride western by the way.