As Prasad D has mentioned that a number of scenarios which triggers system modification.
For example: SIB10 and and SIB11 used of ETWS primary and secondary notification respectively.
MME has interface with CBS (Cell Broadcast System) which in turn has CBE(Cell Broadcast Entity) and CBC(Cell Broadcast Cell). Cell Broad Centre is responsible to send waring message to all the network like 3G, LTE.
I am not much aware about interface between MME and CBC. However, MME sends WRITE-REPLACE WARNING Message to eNode. On receiving this message, eNodeB start to schedule SIB10 and SIB11.