LTE network consists of LTE Radio and LTE core network. LTE Radio node is well known as "eNodeB" while the LTE core network combination of many nodes such as Mobility Management Entity (MME), Serving Gateway (SGW) and Packet Data Network Gateway (PGW) etc. There are many other core network nodes which play major role to provide end to end LTE network connectivity along with handover of UEs to/from various others 3GPP and non-3GPP technologies.
There can be reason of RRC connection reject due to eNodeB or core network. At eNodeB, if there is a radio resource crunch or it is already congested, it responds with rrc connection reject for incoming rrc connection request. Though it does not send exact reason of rejecting rrc connection request message but it add wait time in the rejection. Based on the value of wait parameter, UE waits and re-initiate RRC connection procedure again to eNodeB.
There can be another possibility that MME or some other core network node is overloaded/congested, because of that MME sends Overload Start message indicating that eNodeB should reduce signalling. Assume, If eNodeB is having only single MME connection then in this case eNodeB may stop accepting incoming rrc connection request based on the information received from MME in the Overload Start message.
