Ok, so after some googling and disabling wake on lan - the issue disappeared.
Solution was found ubuntu bugs.
Though it still in the log says 'reboot down', but after disabling wake on lan it doesn't actually reboot anymore. So I think that bug is only partially solved. In other words - it doesn't reboot, but it also should show that log of 'reboot: power down'.
Does anyone know how to report bugs upstream?