The rink announcement was "medical reasons". That's the wording they've used on all withdrawals. It could be illness, injury, or altitude.
The surprising thing was that they didn't make any announcement in the arena during the warm-up. There were just 5 skaters when there should have been 6. Either someone at USFS missed the boat on informing the organizers of the withdrawal or they still hoped at that point that she could skate. I think officially if there's no withdrawal they call the skater's name when it's their turn to skate and then wait a short, fixed period of time to see if they show up. If they don't, they are then withdrawn. Thank goodness they actually got an official withdrawal before her turn came.