It is the case when UE re-selects from UMTS/GPRS to LTE or vise versa.
If you were on UMTS network you will have P-TMSI and after TAU you are camped on LTE cell.
but MME has not assigned GUTI to you.
In this case MME can contact the UMTS SGSN to request the subscribers current profile (IP address, PDP contexts, etc.).