linuxlite and ubuntu gnu/linux installs may have automatic unattended upgrades installed and enabled
to check the status of your system
$ aptitude show unattended-upgrades
if installed you can disable it by editing /etc/apt/apt.conf.d/50unattended-upgrades or remove the unattended-upgrades package
# aptitude remove unattended-upgrades # rm /etc/apt/apt.conf.d/50unattended-upgrades # reboot
In past versions of Ubuntu, unattended package upgrades were handled by creating a cron job in /etc/cron.daily responsible for running apt. With 16.04 and the shift to systemd, unattended upgrades are handled by a systemd unit. The new systemd unit has OnCalendar set to 6am and 6pm, along with Persistent set to true. The result is that a full apt-get update && apt-get ugrade will run the first time the box boots, in order to “catch-up” with missed runs.