Optimizing query on a partitioned table
If you have a composite clustered index on (at least) (column1, column2, column3), then a non-clustered index on column5 is covering for ( column1, column2, column3, column5 ) and would be narrower than the clustered index, especially on a wide fact.  So that might be why.

Can you post the DDL and indexes for your table please?

I'm just going to reproduce that section from "Query Tuning and Optimization", CH3 here as hopefully it will help clear up what looks like a misunderstanding you have about covering indexes:

CREATE TABLE T_heap (a int, b int, c int, d int, e int, f int)
CREATE INDEX T_heap_a ON T_heap (a)
CREATE INDEX T_heap_bc ON T_heap (b, c)
CREATE INDEX T_heap_d ON T_heap (d) INCLUDE (e)
CREATE UNIQUE INDEX T_heap_f ON T_heap (f)
CREATE TABLE T_clu (a int, b int, c int, d int, e int, f int)
CREATE UNIQUE CLUSTERED INDEX T_clu_a ON T_clu (a)
CREATE INDEX T_clu_b ON T_clu (b)
CREATE INDEX T_clu_ac ON T_clu (a, c)
CREATE INDEX T_clu_d ON T_clu (d) INCLUDE (e)
CREATE UNIQUE INDEX T_clu_f ON T_clu (f)




Hopefully that makes senses!