The signalling connection release indication procedure is used by the UE to indicate to the UTRAN that one of its signalling connections has been released. The procedure may in turn initiate the RRC connection release procedure.
The UE shall, on receiving a request to release (abort) the signalling connection from upper layers for a specific CN domain:
if a signalling connection in the variable ESTABLISHED_SIGNALLING_CONNECTIONS for the specific CN domain identified with the IE "CN domain identity" exists:
initiate the signalling connection release indication procedure.
也就是说,一旦UE收到从UE上层来的request to release (abort) the signalling connection消息,它会trigger Signalling Connection Release过程。