LTE Association occurs by exchange of control packets between the ENB and UE.
Initially the UE sends RRC_CONNECTION_REQUEST packet to the ENB.
The ENB in response sends a RRC_CONNECTION_SETUP packet back to the UE.
The association is completed when the UE sends a CONNECTION_SETUP_COMPLETE packet back to the ENB.
This process can be understood using NetSim's packet trace file.
Note: LTE Association procedure happens also after an event of Handover.