8.1.2 postmaster died

One of our 8.1.2 postmasters on HPUX 11.23 ia64 just received a
SIGKILL signal from unknown origins.  After reviewing all
command history files for the DBA and root, I do not believe
anyone manually sent it, and we have no scripts etc that would
do that, at least that we can find or imagine.  The machine had
ample RAM available.

Any ideas where could this SIGKILL would have come from?

Thanks,
Ed