kd,
I assume that when CO saw the reports from the Consulates in the first week of July, he concluded that insufficient applicants were following through with NVC to reach the interview stage to use up the available visas for the year. We can probably ignore AOS because it won't form a large part of F2A approvals (<5% last year). To do so, applicants need to submit what was called packet 4 and NVC need to process it.
Since 95%+ of approvals will come from within the DOS system, I imagine CO is receiving pretty good information from NVC on the number of people who are now processing packet 4.
I would guess he already knows that there will be more applicants documentarily qualified in October than can be sustained by the allocation for F2A in October 2013.
There's some previous history that such movements generate significant Demand.
When the dates for F2A were last moved forward in a similar manner from June 2010 to December 2010, the dates reached 01AUG10 in the December VB, having started at 01DEC06 in the May VB. Following that, over a 3 month period, the dates retrogressed to 01JAN07 in the March 2011 VB.
In the July 2010 VB he used fairly similar language to that used recently.
He then warned of retrogression in the December VB. He then retrogressed in the January VB and warned of further retrogression, which followed in both the February and March VBs.D. VISA AVAILABILITY IN THE FAMILY-SPONSORED CATEGORIES
There continues to be extremely rapid forward movement of most Family preference cut-off dates. This is a direct result of the lack of demand by potential applicants who have chosen not to pursue final action on their cases, or who may no longer be eligible for status. The rapid movement provides the best opportunity to maximize number use under the FY-2010 annual numerical limitations. Should applicants eventually decide to pursue action on their cases it will have a significant impact on the cut-off dates.
That's my best guess anyway.