Datetime Clustered Index Keeps getting fragmented
Fragmentation can still occur on a table with an ever increasing key even if that key is never itself subject to updates.


If the file group is shared with other objects allocations can be interleaved causing fragmentation.
Updates that increase the size of rows can cause page splits.
Deletes can leave pages nearly empty and cause internal fragmentation.


However you say deletes and updates are rare or non existent. So to reach 70% fragmentation within a week I would assume that the cause must be a shrink database task. See Why you should not shrink your data files