Out of memory: Kill process nnnn (postmaster) score nn or sacrifice child

Hello All,

I have a 4 node PostgreSQL 9.6 cluster with streaming replication. we
encounter today the Out of Memory Error on the Master which resulted in
All postres processes restarted and cluster recovered itself. Please let
me know the best way to diagnose this issue.

The error seen in the postgresql log:

2019-02-12 10:55:17 GMT LOG: terminating any other active server processes
2019-02-12 10:55:17 GMT WARNING: terminating connection because of crash
of another server process
2019-02-12 10:55:17 GMT DETAIL: The postmaster has commanded this server
process to roll back the current transaction and exit, because another
server process exited abnormally and possibly corrupted shared memory.
2019-02-12 10:55:17 GMT HINT: In a moment you should be able to reconnect
to the database and repeat your command.
2019-02-12 10:55:17 GMT WARNING: terminating connection because of crash
of another server process
2019-02-12 10:55:17 GMT DETAIL: The postmaster has commanded this server
process to roll back the current transaction and exit, because another
server process exited abnormally and possibly corrupted shared memory.
2019-02-12 10:55:17 GMT HINT: In a moment you should be able to reconnect
to the database and repeat your command.
2019-02-12 10:55:17 GMT WARNING: terminating connection because of crash
of another server process
-----

Error from dmesg on linux:
-----------------------------------
[4331093.885622] Out of memory: Kill process nnnnn (postmaster) score nn or
sacrifice child
[4331093.890225] Killed process nnnnn (postmaster) total-vm:18905944kB,
anon-rss:1747460kB, file-rss:4kB, shmem-rss:838220kB

Thanks & Best Regards
Vikas Sharma