Mysterious power off

My GRX600K just turned itself off, no warning, battery isn't low, no
nothing.  It just went dead *BOOM*.  I didn't try to turn it back on
immediately, I first made sure it was still plugged in/etc -- had
plugged it in to charge the batteries.  It was, so the batteries hadn't
gone dead, and once I hit the power button a few minutes later it turned
right no, no problems at all.

Any way of finding out "WHY" it just turned off automagically?

Thanks,

Jay

PS: Yes ACPI is working, yes my fan is working, and no it doesn't appear
to have overheated temperature wasn't that hot and I wasn't doing much.