Transaction was deadlocked on lock resources

Trying to run an update on table. I get the following

Transaction (Process ID 94) was deadlocked on lock resources with another process and has been chosen as the deadlock victim. Rerun the transaction.

Query looks like this.. any ideas?  

    UPDATE dbo.ae_dt2
    SET
      field3 = Src.Item
    FROM Reps Src, dbo.ae_dt2 Tgt
    WHERE
      Src.RepNo = Tgt.field3