It'll likely be the sort spilling to TempDB. If you look in the default trace you'll see the sort warning in there. Sorts very commonly spill, especially if there're on largish resultsets as the memory grant they would require can be quite large (larger than the total size of the resultset)
What's the definition of the pk?
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability