It looks like the following will work.
SELECT SUM([Net Loss]) OVER( PARTITION BY ADM_RISK_GRADING_ORG_UNIT_ID, ADM_EVENT_TYPE_ID, ADM_CAUSE_TYPE_ID ORDER BY [Effect Date YYYYMM] ROWS BETWEEN 12 PRECEDING AND 1 PRECEDING
FROM dbo.MyTable
Drew
J. Drew Allen
Business Intelligence Analyst
Philadelphia, PA