SQL Server Deadlock on delete
There is the possibility that you are getting lock escalations onto the table level, or page locks on the clustered index page. A deadlock graph will show that, try starting a SQL Server trace grabbing just the Deadlock graphs from the server and check those

You can also delete in batches which will improve concurrency and if the delete is handling large number of rows might run faster and allow you to backup and truncate the log and lower the changes of deadlocking.

DECLARE @ROWCOUNT int = 1
WHILE @ROWCOUNT &gt; 0
 BEGIN delete top (10000) from xx where RowNumber &gt;1
 set @rowcount = @@rowcount
END