I tend to get a sore back after long rides, it doesn't matter what bike I ride, it is made worse if my camelback is particularly heavy.
I have asked around about it and it's to do with my gluteus maximus muscles, because I sit down a lot during the day the muscles are stressed after riding the bike for a while.
Obviously your back pain could be described by loads of things (I'm not a medical expert) but it's worth considering there may be other reasons to your back pain.