check out 3GPP 23.122
NAS Functions related to MS in idle mode
An LR request indicating Normal Updating is made when, in idle mode,
– the MS changes cell while being in the update state NOT UPDATED; (for MS capable of GPRS and non-GPRS services when at least one of both update states is NOT UPDATED)
– the MS detects that it has entered a new registration area, i.e., when the received registration area identity differs from the one stored in the MS, and the LAI or the PLMN identity is not contained in any of the lists of “forbidden LAs for roaming”, “forbidden LAs for regional provision of service”, “forbidden PLMNs for GPRS service” or “forbidden PLMNs” respectively, while being in one of the following update states:
– NOT UPDATED;
– ROAMING NOT ALLOWED.
– the Periodic Location Updating Timer expires while being in the non-GPRS update state NOT UPDATED (triggers Location Updating);
– the Periodic Routing Area Update timer expires while being in the GPRS update state NOT UPDATED (triggers Routing Area Update);
– a manual network reselection has been performed, an acceptable cell of the selected PLMN is present, and the MS is not in the UPDATED state on the selected PLMN.
Furthermore, an LR request indicating Normal Location Updating is also made when the response to an outgoing request shows that the MS is unknown in the VLR or SGSN, respectively.
I would say it has to do with GPRS state…? But as you see, there are several possibility for a LOCATION NORMAL UPDATING.