Osteoarthritis is, as you say, more common in the back (spine), hips and knees. However, it can occur in any joint. It’s called the wear-and-tear arthritis and, if you think about it, you usually do use your thumb quite a bit. And, some people – manual laborers, for example – put a lot of pressure on their thumb and that joint.
As the tissue in the joint begins to wear away, this is what causes the pain.