During SRVCC while Sending MobilityFromEutraCommand you need to configure both PS and CS RABs of UTRAN to make DataSession and Voice Call resumption.
Where as in Case of GERAN either a CS Call or PS Can work at a time. Both CS and PS Calls does not work on GERAN. So for SRVCC to GERAN since Voice needs to be Resume the UE will send GPRS Suspension Request to Stop the Ongoing DataSession