I saw a recent video of him presiding over some service and it was obvious he was really feeble.
It seems like it is just what he said. He is just getting too old and infirm to carry on with his duties.
I think he is an eminently practical man as well as an unparalleled scholar and theologian. I believe he looked back at the final years of JP II with his declining health, and thought it better to step down before age and infirmity took their toll on him as well.
It has occurred to me that he may have cancer and is in pain.