My experience with most serious medical things is that doctors have a clear general-audience explanation, because part of their job is making patients feel like they understand what's going on. But if you get into the details, the good doctors will say, "Well, actually, the picture is much murkier, but here's what we know based on studies X, Y, and Z, but there's a lot yet to learn."
Back pain in particular is an area of open dispute. Many people have many theories, all of them very hard to test. When I was dealing with back pain, a spine-center doctor recommended a book called "Treat Your Own Back" . It was hugely helpful to me. And in it, the doctor explains that he hit upon his method because a patient came in, used a piece of equipment that was set up totally the wrong way, and said, "Gosh, doc, that was great!" It led him to reexamine what he had been taught about backs, coming up with a different "clear physical mechanism" to explain a common class of pain.
That we are still figuring something out does not mean it's psychosomatic. It doesn't mean it's not, but a doctor suggesting otherwise is doing the medical equivalent of "god of the gaps" theology, where anything mysterious is attributed to your preferred cause.