Modify Bearer Request is triggered when
UE sends Attach Complete and activate Default EPS bearer context Accept as NAS PDU in RRC layer UL information Transfer message,which acknowledges the completion of attach procedure and establishment of EPS bearer.
This NAS PDU is forwarded by the eNB in the UL NAS transport message to MME.
Upon Reception of this message,MME sends the Modify Bearer Request to S-GW.This message contains DL TEID and transport address that the eNB provided for default EPS bearer in the initial context response message.
Please correct me if I am wrong.