Staying power
It clearly states in canon 401, paragraph 2: “A diocesan bishop who, because of illness or some other grave reason, has become unsuited for the fulfilment of his office, is earnestly requested to offer his resignation from office.”
This implies that while a diocesan bishop perhaps should resign, and while he may be requested to do so by various interests, or even by the Vatican, he must initiate the resignation process himself.