Pretty much every time it has shut down unexpectedly, I get (41) The system has rebooted without cleanly shutting down first. This error could be caused if the system stopped responding, crashed, or lost power unexpectedly.
Though this is just after rebooting, so it doesn't really give any details about the cause of the problem.
Those are the only critical errors I have in my log, at every reboot.
Just before the shut down, I do have a warning event, two back to back actually for (701) Power Manager has not requested suppression of all input (INPUT_SUPPRESS_REQUEST=0)
Those seem to pop up just a couple minutes before the shutdown actually occurs, that is the only thing I notice to be regularly happening just before the shutdown, that is categorized other than information.