>I think originally this column was not INT but NUMERIC(6,0) and I later changed it to INT. Could this have confused SQL Server and caused the PK column to be non-clustered?

It all depends on how exactly you created that PK originally. By default SQL Server tries to make the PK column a clustered index if you do it using the designer. I don't think numeric(6) vs. integer would have made a difference.

Of course, it's easy to test - just create a new table with few columns and a numeric(6,0) column, set it to be a PK and then see what is the generated script. Then just don't save changes in the designer or if the table will be created, just drop it.