We had a similar thing happen to us
18/07 - NOA2 approved
24/08 - Case was sent back to USCIS
15/09 - Case was reaffirmed and sent back to NVC
25/10 - NVC number issued and case sent to embassy.
Very odd that this has been happening to lots of people. Our lawyer could offer no explanation, other than potentially a clerical error at the USCIS end. Bizarre.