This is the case of handover procedure followed by TAU. Tracking are update procedure is always triggered by UE when TAU list maintained at UE doesn't match with tracking area of current cell. Now coming to handover procedure. LTE supports two types of handovers X2 based handover and S1 based handover.
In case of S1 based handover, MME is already aware about target eNodeB of UE. MME takes decision on SGW change. MME chooses suitable SGW if required based on the target eNodeB. So there is no requirement to execute 3GPP PATH SWITCH procedure.
But in case of X2 handover, MME doesn't aware about what's happening at eNodeB for an UE in terms of handover. Once handover successfully gets executed, target eNodeB sends PATH SWITCH REQUEST message to MME for that UE. PATH SWITCH REQUEST message contains IEs (ECGI, TAI etc) which helps MME to select appropriate SGW. If there is no change is SGW then MME executes Modify Bearer Procedure else MME creates bearer with new SGW for UE by executing the Create Session Procedure and responds to eNodeB with PATH SWITCH REQUEST ACKNOWLEDGE message.
UE, after sending the RRC Connection Reconfiguration Complete message to target eNodeB, it can perform Tracking Area Update Procedure if the target eNodeB belongs to Tracking are area which is not present in the TAI list. MME can reject TAU request sent by UE due to various reasons. One of reason might be Tracking Area not allowed.