As a relatively young helicopter pilot, I have had two back surgeries to repair herniated disks. No proof, but I am fairly sure that the poor cockpit ergonomics and vibrations are to blame.
I regularly fly two different cockpit types- one has a sidestick cyclic and forces me into a good sitting position (no pain after long flights), and the other has a center cyclic and forces me into a forward lean (discomfort after even short flights).
The problem is that customers (military and civilian) do not insist on good cockpit ergonomics for the pilots. When it comes to accepting cost cutting measures, the pilot's seating position is the first one that seems to go.