When an UE switches it's connection state from RRC-connected to RRC-IDLE, eNodeB informs to MME by using S1-AP based "UE Context Release Request" message MME. As soon as MME receives this message from eNodeb with cause "due to user inactivity", it initiates "Release Access bearer" procedure with SGW.
The objective of sending this message is to just inform SGW that UE is not any more at that eNodeB. Please release the associated with locally and if any data comes from the external network for that UE, just inform me(MME).