The short version:

Blood oxygen levels were falling into "should be dead" levels never really seen before (even though they were otherwise alert and fine and had no other hypoxia indicators), which caused doctors to panic about pneumonia and not being able to breathe, and put people on ventilators prematurely. However, it wasn't really having an effect, so the ventilators kept getting turned up to their maximum setting, which puts too much pressure on the lungs and causes additional permanent damage. In most people this caused a turn for the worse instead of recovery.