I would suggest that it is to do with dropping unused partitions in a table. Before doing this though, you should be careful to identify whether or not the indexes you have on the partitions are local or global indexes. If the indexes are global to the partitioned table, after the partition is dropped, the associated table index will marked 'unusable', until it is rebuilt.
Anyone else with thoughts on this?
Cheers
OCP 8i, 9i DBA
Brisbane Australia