why does my server sometimes reboot by itself?

Hello

I use openbsd3.9. normally it works well. the server uses its own default
kernel. I have not changed concerning kernel . But My server reboots by
itself. it gives an error as below;

Jun 27 22:23:16 opendns /bsd: panic: trap
Jun 27 22:23:16 opendns /bsd: Stopped at        Debugger+0x4:   ret
zero,(ra)
Jun 27 22:23:16 opendns /bsd: RUN AT LEAST 'trace' AND 'ps' AND INCLUDE OUTPUT
WHEN REPORTING THIS PANIC!
Jun 27 22:23:16 opendns /bsd: DO NOT EVEN BOTHER REPORTING THIS WITHOUT
INCLUDING THAT INFORMATION!
Jun 27 22:23:16 opendns /bsd: ddb> ddb> ddb> No such command
Jun 27 22:23:16 opendns /bsd: ddb> print       examine     x           search
set         write
Jun 27 22:23:16 opendns /bsd: w           delete      d           break
dwatch      watch
Jun 27 22:23:16 opendns /bsd: step        s           continue    c
until       next
Jun 27 22:23:16 opendns /bsd: match       trace       call        ps
callout     show
Jun 27 22:23:16 opendns /bsd: boot        help        hangman     dmesg
Jun 27 22:23:16 opendns /bsd: ddb> syncing disks... 8 8 5 done
Jun 27 22:23:16 opendns /bsd: rebooting...
Jun 27 22:23:16 opendns /bsd: rebooting...
Jun 27 22:23:16 opendns /bsd: [ using 507032 bytes of bsd ELF symbol table ]
Jun 27 22:23:17 opendns /bsd: consinit: not using prom console

What might this condition be cause ?

What shall I do ?

Thanks