why and when does the parents would send the leave NWK CMD to its children？

Hi all

         there  is  a ZED which is the child of  the ZR。

         when aging some time later ， i  found  that  when the ZED send something to ZC through its parents ZR，  ZR would send a Leave  NWK  CMD  to  ZED and have not pass the message to ZC.

         But ZED could not  notice it is already disconnect with its parents at  this moment。

         does anybody konws why ？