Alex clearly has had enough. She's in pain and no longer enjoying it. I suspect conversations have been had behind the scenes where the powers that be have begged her not to drop out injured (they can't afford any more injury withdrawals this series). Probably they have assured her that if she's in the dance-off this week the judges will let her go with dignity in order to avoid losing her through withdrawal and finding themselves with not enough finalists.
Loving Karen and Carlos, I think if she can keep the momentum she could win the whole thing as the "journey" contestant, and Carlos's first win would be a good narrative. The only problem is that she has used up most of her good/fun dances which suit her and has a bunch of tricky ones left.