The system is powering off because `AutoPowerOff` is enabled.  

To disable it, issue the following command in Terminal:

    sudo pmset -a autopoweroff 0


That command will set `autopoweroff` to "disabled" for all profiles (`-a`).  This means it covers you when on battery and on A/C.  If you just want `autopoweroff` disabled when on battery, use the following command:

    sudo pmset -b autopoweroff 0


* `-b` = battery
* `-c` = (wall) charger
* `-u` = UPS
* `-a` = all profiles

See [`man pmset`][1] for more details.


  [1]: https://developer.apple.com/legacy/library/documentation/Darwin/Reference/ManPages/man1/pmset.1.html