Head movement causes pain in the back of the neck. It has been said that it is caused by a bulging disc, due to osteoporosis.

There are lots of things that can cause neck pain. Why guess? If I were you, I'd go have it imaged since treating muscle pain and a bulging disc require different treatment strategies. A bulging disc can't be messed with or you risk making it worse. Don't go to a chiropractor and have it manipulated without knowing the facts, and this means getting it imaged and having a discussion with an orthopedist.

I think osteoporosis has nothing to do with a bulging disc.

What is your question? If it "has been said that it's caused by a bulging disc", has it also been said what the remedy is? I had a similar situation in 2008 which required neck fusion surgery at C3 thru C5. I've had no pain since the surgery.