As I understand it, remdesivir doesn't directly cause respiratory failure. What it does is cause KIDNEY failure, such that the kidneys can no longer remove waste fluids from the blood. Since the cells in the lungs are permeable, those fluids collect there instead, and the patients essentially drown when said fluids prevent the lungs from absorbing oxygen.

That's why hospitals began rolling patients onto their stomachs, to try to avoid the pooling in the lungs, and facilitate draining (or vacuuming) out the mouth.