You mean bedsores, yeah its a pretty similar cause. The difference is that bedsores only cause damage to the skin from lack of blood flow caused by the extended pressure from lying down. While in horses, the extended pressure from lying down leads to poor blood flow in not just the skin, but the muscles and organs nearby too.

And yes, you can turn horses over too, but it takes multiple people and is really dangerous to everyone involved, so its not an action to be taken lightly.