what could cause postgres to crash?

Hi,

My postgres sessions, after being idle for 5 --6 hrs, crash on their own.
Sometimes with error messages sometimes without. The message I get appended
below. I was looking for suggestion to narrow down as to what could have
caused this problem. System log doesn't show anything.

Thanks.
Sandeep

LOG: statistics collector process (PID 6631) was terminated by signal 9:
Killed
LOG: server process (PID 15710) was terminated by signal 9: Killed
DETAIL: Failed process was running: COMMIT PREPARED 'T13199'
LOG: terminating any other active server processes
WARNING: terminating connection because of crash of another server process
DETAIL: The postmaster has commanded this server process to roll back the
current transaction and exit, because another s
erver process exited abnormally and possibly corrupted shared memory.
HINT: In a moment you should be able to reconnect to the database and
repeat your command.
LOG: all server processes terminated; reinitializing