There was no indication about why the pope felt it necessary to release the document, particularly since his 2000 document summed up the same principles. Some analysts suggested it could be a question of internal church politics, or that it could simply be an indication of Benedict using his office as pope to again stress key doctrinal issues from his time at the congregation.

I'm opining that he's just a John Roberts fan, similarly taking back the helm and setting sight on a more familiar point. No need for a 5-4 vote here:

It was the second time in a week the pope has corrected what he says are erroneous interpretations of the Second Vatican Council, the 1962-65 meetings that modernized the church.
...
Benedict, who attended Vatican II as a young theologian, has long complained about what he considers the erroneous interpretation of the council by liberals, saying it was not a break from the past but rather a renewal of church tradition.

In the latest document — formulated as five questions and answers — the Vatican seeks to set the record straight on Vatican II's ecumenical intent, saying some contemporary theological interpretation had been "erroneous or ambiguous" and had prompted confusion and doubt.