At the high level, there could be two reasons when UE establishes RRC connection with the eNodeB.
Signalling
When an UE wants to send signalling information to MME or other nodes and as result there is no data exchange between UE and network , comes under the establishment cause - "Mobile originating signal".
Example: Attach Request , Detach Request, TAU procedures comes under the "mo-signalling".
Data
When an UE establishes rrc connection with network and as result data exchanges between UE and network, comes under the establishment cause - "Mobile origination data".
Example: Service Request, Extended Service Request comes under the "mo-data".
