She downgraded, too, did she not? I swear she did a full twist in prelims.
My guess is that its been slowly falling apart and when she hit the springboard it tore enough, but probably not completely, for her to know something was wrong, hence bailing and only doing a half–an easier landing if you fear your knee is compromised. Then the landing was just too much when her ACL was already destroyed.
Very similar thing happened to me. I kept training through persistent but tolerable knee pain and instability until there was no ACL left. It just kinda unraveled slowly over time. My knee didn’t even buckle like Asia…I just finally decided it wasn’t getting better and needed see a doc. We were all shocked when the MRI results came back.