My husband and I both have back pain in the same area

1. Sagging, poor quality mattress
2. Sitting at a computer all day, not realizing that you lean slightly to the side away from your mouse hand, putting weight on that side
3. Running along a road is bad because roads are sloped down on the sides and your feet 'jar' as you hit the higher side
4. Carrying a child on one hip
5. Coincidence - you have different reasons from each other

There are countless possibilities, of course. I listed a few common ones.