What operations set CTXSYS.CTX_USER_INDEX_PARTITION.IXP_STATUS to 'NO_INDEX'?

We have a simple LOCAL CONTEXT index on a partitioned table in an 11.2.0.3 EE database.  If I create the index UNUSABLE, all the Text partitions have IXP_STATUS = 'NO_INDEX' and no underlying DR# objects are created.  No problem...I just alter rebuild each index partition.  However, in one of our test databases, I see such a partitioned table with 200 out of 2,500 of the index partitions set to NO_INDEX while all the others are INDEXED.  These 200 partitions were created a year after the original Text index partitions were rebuilt and became USABLE, and there are other USABLE/INDEXED partitions