My guess is he is getting senile, maybe Alzheimer’s. If he had a terminal illness he would just let it run its course and have a new successor. If he was just frail, then he would cut back on touring.
I fear he’s losing his mental faculties, and doesn’t want to linger on for years not even knowing who he is, leaving the Church paralyzed.
Just a guess as to why he would do such an unusual thing.
It would not surprise me that it could be because after seeing how Blessed John Paul II suffered bigtime from both age and ill health while doing his duties as Pope, Pope B16 felt he could not subject the Church once again to another aging and suffering Pope.
Plus having written 3 best selling books about Jesus must having its toll also on him.