1) The MME knows the services for which the UE is subscribed to from HSS. If the UE sends Service request for the services for which it has not subscribed to, then MME will send a service reject to that particular UE. For example : UE is Subscribed for Voice services only, and it sends a service request for Data. This causes MME to send service reject. one scenario is this
2) Another scenario could be overload conditions Where MME is overloaded and it cannot serve any other Requests from UE.
Hope I have answered the question.