It is not necessary that UE has requested some data from IP world. Usually when there is no data transmission happen for an UE for a configured time (user inactivity timer) at the eNodeB, RRC Connection Release is triggered for that UE .
Once the PGW receives an IP packet for an UE , it forwards the packet to SGW and since SGW doesn't have tunnel towards eNodeB for that UE, it sends downlink data notification. As results, MME does paging for that UE.
Please capture and analyze the wireshark logs at PGW . Verify from IP traffic is coming for that UE by using the filter.
This will help you to find out the exact reason .
I tested data path using the iperf application. Once the UE goes into idle mode and after that I pump data for iperf . MME does paging .