Why two bishops resigned

This morning, the Holy Father accepted the resignations of two bishops from the Archdiocese of Minneapolis and Saint Paul (USA).  The official announcements read as follows:

The Holy Father has accepted the renunciation of the pastoral leadership of the Archdiocese of Saint Paul and Minneapolis (USA), presented by His Excellency, John Clayton Nienstedt, in conformity with the provisions of Canon 401, paragraph 2 of the Code of Canon Law. 

The Pope has named as Apostolic Administrator 'sede vacante' of the Archdiocese of Saint Paul and Minneapolis, His Excellency, Bernard Anthony Hebda, Coadutor Archbishop of Newark.

and

The Holy Father has accepted the renunciation from the office of Auxiliary of the Archdiocese of Saint Paul and Minneapolis (USA), presented by His Excellency, Lee Anthony Piche, in conformity with the provisions of Canons 411 and 401, paragraph 2 of the Code of Canon Law.

The first clue about the motive for the resignation of a bishop is immediately apparent by the Canons that are quoted when the notice is published.  Canon 401 of the Code of Canon Law (1983):

Can. 401 §1. A diocesan bishop who has completed the seventy-fifth year of age is requested to present his resignation from office to the Supreme Pontiff, who will make provision after he has examined all the circumstances.§2. A diocesan bishop who has become less able to fulfill his office because of ill health or some other grave cause is earnestly requested to present his resignation from office.

The fact that paragraph 2 was quoted for both of the above resignations means that there is more to the story.  As it turns out, much more.  By accepting two resignations from bishops in the same Archdiocese on the same day, the Pope wants to make very clear that he is serious about the words he has spoken regarding the importance of protecting minors, including a new judicial section within the Congregation for the Doctrine of the Faith to deal with cases of Bishops accused of covering up for priests who have abused children, and Pontifical Commission for the Protection of Minors, which is headed by Boston's Sean Cardinal O'Malley, OFM Cap.