I have a theory... a bit far fetched perhaps, but think of it this way. Perhaps the producers had more input than was announced? I see it going like this... CdlF can't keep going and needs the surgery. The producers know that Shannon has the lowest votes. They have to decide whether to let Shannon go or not. Given the negative publicity associated with Shannon/Derek's "relationship", the producers decide that they must go and tell CdlF that he has to say he's coming back with the full expectation that he won't make it through the week and will "retire."
What do you think?