Unexpected shutdown

As I was working with another computer, not doing anything actively besides sharing mouse and keyboard using barrier, my desktop unexpectedly initiated shutdown.

In syslog, these are the very first messages of the time:

Jun  7 20:10:42 albert mate-session[22164]: WARNING: Unable to find desktop file 'keepassxc.desktop': Impossible de trouver un fichier de clés valide dans les répertoires de recherche
Jun  7 20:10:42 albert mate-session[22164]: WARNING: Unable to find desktop file 'mate-keepassxc.desktop': Impossible de trouver un fichier de clés valide dans les répertoires de recherche
Jun  7 20:10:43 albert systemd[1]: Stopped Unattended Upgrades Shutdown.
Jun  7 20:10:43 albert systemd[1]: Stopped QEMU KVM preparation - module, ksm, hugepages.
Jun  7 20:10:43 albert systemd[1]: session-c2.scope: Killing process 2036 (pulseaudio) with signal SIGTERM.
Jun  7 20:10:43 albert systemd[1]: session-c2.scope: Killing process 2054 (deja-dup-monito) with signal SIGTERM.

Then I have a lot of messages for the ongoing shutdown.

In auth.log, I see

Jun  7 20:10:42 albert polkitd(authority=local): Unregistered Authentication Agent for unix-session:c4 (system bus name :1.3119, object path /org/mate/PolicyKit1/AuthenticationAgent, locale fr_FR.UTF-8) (disconnected from bus)
Jun  7 20:10:43 albert lightdm: pam_unix(lightdm:session): session closed for user david
Jun  7 20:10:43 albert systemd-logind[893]: System is powering down.

I did not press the power button ad did not ask to log out.

From syslog, it looks a bit like mate-session could be responsible for the shutdown. The message in French says something like "Unable to find a valid key file in in search directories", the "key file" might refer to the gnome keyring rather than keepassxc, but I don't know.

I welcome any clue.

Besides, my desktop was fully stable for several weeks, until yesterday, since then I lost control of the mate session at least three times, I managed to close it via ssh either by restarting lightdm or using mate-session-save.