Subject Strange server shutdown


i have encountered a strange behaviour with a firebird server 3.02 on debian (9 strech) in superserver mode :

the server has shutdown without any reason (no human).

in fb log i can see that : 

Wed Mar 21 18:01:12 2018

Shutting down the server with 736 active connection(s) to 91 database(s), 0 active service(s)

Wed Mar 21 18:01:16 2018

/usr/sbin/fbguard: /usr/sbin/firebird normal shutdown.

the last log say that is a normal shutdown but i don't see why (noting in other linux log).

what are the possible case for a normal shutdown ?

An other thing strange is that fbguard doesn't restart the fbserver (GuardianOption = 1 in firebird.conf)

someone have an idea about what could have happened ?