Hi Charan,
First thing is that Even though UE is in IDLE mode , Bearers(Contexts) will be exist from PGW to SGW and SGW to MME, But From SGW to eNb and MME to eNb won't exist.
So when ever data comes to the PGW, then It will map that data to the bearer by using PCC rules and sends to the SGW. Now in the case of UE IDLE mode SGW can't find the eNb details to send, then SGW will Initiate the DL data arrival indication.
Thank you.