When Assign Master items to child organization , run DELETE FROM DR$WAITING , why

Hi experts

In 12.1.3.

In master items form, assign item to child organization , the performance is poor. Generate trace file see that following SQL take more time. Why assign item to child organization need to run following script?

SQL ID: 1xjwug6sa5g2d
Plan Hash: 1546474621
DELETE FROM DR$WAITING
WHERE
 WTG_CID = :B2 AND WTG_PID = :B1 AND EXISTS (SELECT 1 FROM DR$PENDING WHERE
  DR$PENDING.PND_CID = DR$WAITING.WTG_CID AND DR$PENDING.PND_PID =
  DR$WAITING.WTG_PID AND DR$PENDING.PND_ROWID = DR$WAITING.WTG_ROWID)

call     count       cpu    elapsed       disk      query    current        rows
------- ------  -------- ---------- ---------- ---------- ----------  ----------