I was just troubleshooting a long-running query that happened to be an UPDATE statement and found that a Clustered Index Update operation was a good portion of the total query execution plan. Unfortunately, the table I'm updating has a composite primary key that is also the clustered index. However, the statement was not updating any columns in the key.
So why would there be a Clustered Index Update operation? Simply put, the clustered index is the table. What I mean by this is that table data is stored alongside the clustered key with this key dictating the order in which the rows are stored. Hence, any update to a table is an update to its clustered index.
Also, keep in mind that a clustered index update that affects the clustered key also has to update the all non-clustered indexes since all non clustered indexes reference the clustered key. However, this is not applicable to the above scenario as my UPDATE statement was not updating the clustered index/key.