He could very well be having health issues that we do not know of, who knows.
British TV is saying the vatican has said it is because of ill health.
His brother in a statement confirms the Pope’s bad health as the reason, saying “Das Alter drückt”, i.e. that the pressure of his age has caught up with him.
Of course one can speculate: Is it cancer? Does he know / feel that he will die soon? Or has it just been opportune timing to best position the successor he has in mind?
Many expected JPII to resign because of health and there were sound reasons to do so (with all the insecurity a moribund Pope causes for the leadership of the church). He chose not to do so in order to set an example. Ratzinger was a close ally of JPII and his role was crucial during the time of JPII’s illness. I think both decisions deserve respect. Albeit for different reasons (to show that illness and death is a natural part of live - vs. - to show that the office and the church are bigger than any one men).