Estimated Execution Plan SQL Server Sort?
Group by Abandoned_ID, WorkType_ID , Site_Type_ID, time_id


One way to implement a group by is to sort the input. Stream Aggregate Showplan Operator:


  The Stream Aggregate operator requires input ordered by the columns within its groups. The optimizer will use a Sort operator prior to this operator if the data is not already sorted due to a prior Sort operator or due to an ordered index seek or scan.


QED. 

Further reading: Craig Freedman blog entry on Stream Aggregate and the alternative Hash Aggregate.