Could not shut down DB

I could stop it with:

pg_ctl stop -i i

but before that:

May 30 07:21:00 appserver postgres[65202]: [5-1] FATAL: the database
system is shutting down
May 30 07:21:00 appserver postgres[65203]: [5-1] FATAL: the database
system is shutting down
May 30 07:21:00 appserver postgres[65206]: [5-1] FATAL: the database
system is shutting down
May 30 07:21:02 appserver postgres[65221]: [5-1] FATAL: the database
system is shutting down
May 30 07:21:03 appserver postgres[65222]: [5-1] FATAL: the database
system is shutting down
May 30 07:21:04 appserver postgres[65223]: [5-1] FATAL: the database
system is shutting down
May 30 07:23:25 appserver postgres[39643]: [7-1] WARNING: terminating
connection because of crash of another server process
May 30 07:23:25 appserver postgres[39643]: [7-2] 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.
May 30 07:23:25 appserver postgres[39643]: [7-3] HINT: In a moment you
should be able to reconnect to the database and repeat your command.
May 30 07:23:25 appserver postgres[39643]: [7-4] CONTEXT: COPY
sdi_manageditems, line 1

It happened in the weekend, and the server log does not contain anything
except these "FATAL: shutting down" messages, so I'm not able to tell
when is started. Any idea what could possible cause this error?

Thanks,

Laci