@kentuckycat - I have actually had experience with heterotopic ossification, even though it wasn't nearly as serious as it could be in certain cases.
I was in a car wreck that did quite a bit of damage to my shoulder and elbow. It was enough that I needed to have surgery to repair the bones. After the surgery, I went back for a check-up and the doctor noticed some extra growth around my elbow. She said that it wasn't common, but it wasn't uncommon, either. They gave the bone several weeks to heal, and then I had to go back and have the extra ossification removed.
At least in my experience, the extra bone was formed because my body went overboard trying to repair the damage. I think the majority of cases are caused because of some sort of damage to the bone, and I've read that men are more at risk than women. My doctor also mentioned the same thing as the article that the problem is much more common in the hip.
I hope that helps to clear things up for you.