I would think it could be an improper bike fit. Or...you are doing a lot of climbing. I rarely get back pain even when I sit most of the time... I just get some @ss bone pain eventually. The only time I did get back pain was when I went to some trails that required a lot more uphill climbing than I was used to...that seemed to be straining my back more. I have heard people getting back pain that ride bikes that don't really fit them as well as they should. Have you tried any other bike styles or adjusted your bike so see if helps?
-xta