Mail server reboot while upgrading ports

[Dragoncrest]

	Was just going about doing an upgrade to my primary mail server yesterday 
and right in the middle of it doing its thing I hear a beep, it stops, then 
reboots.  No warnings, no errors, nothing.  Just reboots.  Tracking it back 
I was somewhere in the middle of upgrading /usr/ports/dns/p5-net-dns when 
it burped.  It's done that to me before with python upgrades on a different 
box.  I couldn't see what was actually happening at the console as I was 
across the room ssh'ed in when it did this.  Logs don't show anything sadly 
enough and for some reason I can't get the box to log portupgrade or make 
so I can see when errors like this might happen.  Anyone got any ideas what 
might be happening?  I did get the port upgraded, but not before having to 
go in and clean up the mess that this reboot left.  I can't say this was 
necessarily caused by the dns port I was upgrading cause it's done it in 
the past a couple of times with other ports nearly the same way.  Yet after 
it reboots if I go in and install the port manually (usually have to as the 
port gets trashed because of whatever it is that reboots the machine) to 
fix it, then everything plays nice.  I have no idea what's up.  Anyone have 
any thoughts on this?